I'm wondering how much DM/player consultations you engage in surrounding plot points that grow out of PC backstories? Do you and the player plan everything together? How much do you leave for a surprise factor, if anything?  I've read a lot about how important it is to involve players in the storytelling of their own characters, but I'm not sure where that line gets drawn.

If I have a player whose character backstory involves a missing sister, do I consult with them about what exactly has happened to their sister, what she may have been doing, and where she might have gone? Or do I take the core idea and build a story around that for them to discover as they go?

In another case, a backstory involves having stolen a dead 'do-gooder' servant's identity and gone off to try to build a 'heroes for hire' mercenary guild in another land. Do I ask them if they feel having people look for them is part of the story (from the family's perspective, their child is gone. From the servant's family, someone killed their child, and now a lesser nobleman is missing), or do I just send people out looking for them and have them come across wanted posters or private detectives and such.

I have ideas, but I don't want to usurp player's own stories if they have them.

Quote from CuraAnimarum >>

I've read a lot about how important it is to involve players in the storytelling of their own characters, but I'm not sure where that line gets drawn.

1. I'm wondering how much DM/player consultations you engage in surrounding plot points that grow out of PC backstories?
2. Do you and the player plan everything together?
3. How much do you leave for a surprise factor, if anything? 
4. If I have a player whose character backstory involves a missing sister, do I consult with them about what exactly has happened to their sister, what she may have been doing, and where she might have gone?
5. Or do I take the core idea and build a story around that for them to discover as they go?
6. In another case, a backstory involves having stolen a dead 'do-gooder' servant's identity and gone off to try to build a 'heroes for hire' mercenary guild in another land.
7. Do I ask them if they feel having people look for them is part of the story (from the family's perspective, their child is gone. From the servant's family, someone killed their child, and now a lesser nobleman is missing),
8. or do I just send people out looking for them and have them come across wanted posters or private detectives and such.

I have ideas, but I don't want to usurp player's own stories if they have them.

 
Forgive my restructuring things to get to the questions specifically.

1 - For me, the line lies in the simple understanding that as the DM, the World/Setting and everything around it more or less is my job, that those people and beings and events and places are basically my characters.

2 - I take a lot of points, recognizing that what the player describes is from the Character's Point of View (the old "my sister was brave and was taken away!" is how they see it, but the Sister was "I was scared and ran away"). That is, I don't plan, but I try to get as much info about the NPC as possible. Character backgrounds are a huge part of character creation for us, where I help them learn about the place they choose to come from, and don't tell them what they can do, but guide them so they have a better of understanding of why they can and cannot do things.

3 - All of it. But that may depend on the table. When I ran open games, there would occasionally be folks who didn't like that. But also, I don't always go for the easy thing. "my sister had a horrible experience that shaped me as a person" is the easy thing. Immediately I am thinking of a whole imbroglio with the sister, right? But *after* the sister they met someone who taught them, who pushed them, and I would pick *that* instead. An old teacher, an old annoyance, a bully or a kind person. I break backgrounds down into three parts: 5 to 9, 10 to 15, 16 to 20. I ask about the important people to the characters in each of those three periods (which amount to Learning Self, Learning World, Learning Place).

4 - Just what their character saw, understood, and thought. Not what actually happened, but what and how and why the PC saw and thought about it. You do have to drive that point home, though --that it is their interpretation of events.

5 -- Once you have the above, then you develop it according to the way that your world works, using what seems likely for that person in that world in that time. You might use some of it, but all of it has to at least *start* from where the Player describes things.

6/7/8 - Yes to all of it. That's at least Two groups with different reasons for seeking them, not always in line with one another, often at cross purposes. each group will have different goals and motivations and ideas.

it also depends on how and what you want to use those backstory hooks for -- is it a side quest, is it a way to get them involved in the storyline, is it a downtime activity, etc. The intent you have in using it matters, because that will shape how it is set up.

THe sister was kidnapped by someone who made it seem like she went willingly, and that same person tricked anther individual into thinkin they killed a nobleman -- but the villain didn't expect that person to take their place, and now fears that they will reveal the truth about them having kidnapped a sister and keeping her locked away because she has within her the Heart of a Lioness, a rare and mythical property of the blood. If revealed, they won't be able to unleash the homonculi they have been creating using a mixture of bloods as their minions through which they will do something nefarious.

but the sister wasn't the only one with the Blood of the Lioness...

I actually don't consult with my players very much, other than to ask if they'd be open to a backstory arc. What I do, however, is ask where they want their characters to go - what growth or mechanical builds they want in the future. Then I plan things to surprise them based on those insights.

Do you have to collaborate with players? No, but asking questions about what they've imagined can really help with the creativity and plot. Co-worldbuilding can be a wonderful opportunity for your players to delight you with the way they color your game world. And the best part is that you aren't obligated to use any of it. The game world belongs to you. And sometimes subverting or twisting expectations can be just as fun as giving players what they've imagined.

I don’t tell them anything as an DM, and as a player, I don’t want to be told. 
I had a backstory once of being an orphan (highly original, I know) but my DM threw in that the head of a rival group was my long lost sister. I would never have thought to do that. And not knowing about it made the reveal much more interesting both for me and for the rest of the table. 
That also illustrates another reason not to tell. You can make character-specific NPCs relevant to the main plot. It helps keep the overall story more focused, instead of having to run three towns over to resolve a personal plot line. And, if you don’t say anything, then you don’t risk giving away the plot.

Players still have a lot to do this way. In my example, it was up to me to decide how my character reacted to the news, and to discuss options with the party. I very much felt I was driving my character’s story. 

I tend to talk to my players a fair bit and often try to get a bit of detail about backstory points I may try to bring in.  To me, it's important to know what they character knows, remembers or experienced as part of an event or some such from their backstory.  I feel better knowing that when I bring it in, I am not misrepresenting what they player tried to put out there in their background.  

So far as surprises go, I try to surprise my players as often as possible and some random element from their backstory popping up out of nowhere tends to do that.  Maybe a guard of some faction bears a tattoo that matches one an antagonist from their backstory had.  Maybe a lost relative (as mentioned above, where the orphan uncovered a sister they never knew existed) wanders into the tale.

Your specific scenarios seem to want a little more detail, I think.  Knowing a bit more of how the sister disappeared would be integral to building on it, because a kidnapping would likely lead down a very different path than a runaway would.  The identity theft is the same, were people suspicious of the situation, or did they just take it a face value and move on?  In any case, if the player hems and haws at all, I would offer to take that uncertainty off their hands if they want.  The sister may have disappeared and lots of rumors about how and why but no firm answers (at the time) were known.  The stolen identity may have been "closed" but pops up when someone from that scenario sees the PC and recognizes them.

My only reason for seeking some clarity on vague backstory details is to NOT rewrite what the player had in mind when they wrote the backstory.  Without clarification, you might introduce the sister, saying she had escaped her kidnappers, only to discover the player had meant she ran off with a group of folks and never came back.  Misrepresenting the players story runs the risk of creating conflict OOC that you didn't intend.  A few questions should be sufficient to get a better feel for how THEY feel the issue went at the time, and allow you to pick up that situation and turn it to your needs smoothly and effectively, while maintaining the consistency of the players work.

None really. The campaign is their story. A bit of backstory is great and if the player wants to tie it in at moments that work all the better. But generally I’m too busy to track it all :D. Let the campaign be their story. 

I collaborate with my group quite a bit  on their back stories.  It allows me to write  personal arcs for the characters fitting them into the overall campaign  so while the group is striving to do <insert major plot point here>  Something personal is also happening  that will affect one of the characters.  My group also has helped create the setting in which we play.  The pantheon of Deities, the names of the cities and kingdoms, and Historical lore for the setting  they have had input  

How much you want to or don't want to is entirely up to you and them there really is no right or wrong answer 

I mean level 1 or level 3 what is the backstory? You have like 50 gold tops. It’s just wanderlust/exile/thirst for glory. I can’t tie them into some sort of overarching story that hasn’t been told yet.

Thank you for all of this. It's very helpful. I didn't provide a full background for the two examples I shared in my original post, mostly to conserve space. Most of the players have quite a few details. In the second example I used, the PC comes from a mid-ranged merchant family with some ties to lesser nobility. Growing up, a local boy was known for doing good deeds and thought of quite heroically, which this guy scoffed at and had a lot of contempt for. He came upon the do-gooder getting roughed up by some criminals, stood back and watched them kill him, then decided he could show the world what a real hero looks like, stole his clothes and his identity, and left town.  That BG is the most fleshed out, and based on your feedback, I feel comfortable just running with a "consequences for your actions" sub-plot.  

We're playing LMoP, and it's easy enough to tie in some individuals coming to town either from his family looking for him or the family of the dead guy, thinking the PC is responsible for the murder (since he vanished so quickly at the same time).  I've already suggested that it was a branch of the Redbrands in Waterdeep (where he came from) who killed the guy (the PC recognized the cloaks). He's also befriended Droop, not knowing that Droop is one of the campaign's doppelgangers. Droop keeps making comical and sarcastic backhanded compliments and remarks about  'real heroes'.

The other PCs have some good seeds, but based on the advice here, I do need to ask a few more clarifying questions.

I encourage my players to contribute to the world.  Whether that is a backstory, PC goals, etc.  They are not very consistent in doing so though so I find myself writing 95% of everything.  That being said, it isn't always a bad thing to be in the diver's seat as the ideas flow better and are congruent to your world.  When they do contribute, I retain the right to edit what they thought of so it won't throw anything out of kilter.  

I feel like "How cool are you with me riffing on plot hooks in your backstory" is probably a good Session Zero topic, but for a game in progress... I'd say there's merit both to collaborating and to making it your responsibility, so I suggest maybe quickly nudging the relevant players like "Do you have a lot of ideas for where you want this to go or how important you want it to be, or should I surprise you?" Maybe just ask the whole table so they don't get any ideas about whose backstory you're looking to incorporate or how soon, and so you're giving everybody the same chance to make their preferences known. And if they say they do want to collaborate, you could talk more in private later.

I'm fairly open with my players at first, to get the lie of the land and how comfortable they are with me using their backstories.  I have one character who has a pretty developed backstory, so while I have some surprises for her in how it'll affect the story going forward, I've been very open with anything her character would already know, and shared things with just her so she can make any alterations to it as she sees fit for her character.  However, I have another player whose character basically knows her home was destroyed, and by a creature, but she doesn't know what.  So I've totally run with it, and I'm giving her surprise dreams as I drop hints about what sort of creature it was... (I'm running Dungeon of the Mad Mage, so I'm tying it into Umbraxakar's story...)

I certainly avoid giving away spoilers or giving hints for what is coming (outside of hints their players may get in the course of events) but I try to be pretty open and have a give in take with my players as far as what they are liking, what they aren't liking, what they think or hope is going to happen, etc. It helps me tweak and adjust the story as things go along to create a more fun experience for everyone. I also try to incorporate players backstories in events where I can so I will have discussions with players specifically around their back stories and events that are happening to flesh those out so the can RP them more satisfactorily. As the DM I am ultimately the guide and architect of the story, but I have always seen it as a collaborative effort with the players to create and develop a fun and enjoyable narrative, so I like to have a good feel and understanding for what my players are feeling and hoping for to help guide that.