So, I DMed a Curse of Strahd campaign a while ago. We only got to the second town and the campaign fell apart after that (because of ing-game and RL problems). One of my players said that he really enjoyed CoS and that he would like to borrow the book, so that he can run it. He invited me to this campaign. I also enjoyed CoS, but I just had (and have) too much going on ATM to DM a campaign.  Now, if I would play in this campaign, how would we go about that? I have read the module and know the important plot-points. Should I refrain from playing? I thought about making a dumb character, to make it easier not to meta-game. thoughts? Is this a bad idea?

I will sometimes roll my own history check.. if I am confused about how much what I think my PC knows is mixed with my own knowledge, or what I think common knowledge will be.

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Put yourself in the mindset of the character, they wouldn't know anything about things even if you do.

Just don't meta game/reveal things that you shouldn't know.

Every DM runs campaigns differently, even if it's from the same book, every party picks different paths/choices. 

Your DM might add in things from CoS, or might take things out they don't care for. I'd say play the game, enjoy yourself, and don't point things out if the DM changes it from how it was in the book, or that the other players "aren't doing the thing correctly".

Just play it and don't meta. If you think a situation is too difficult for you not to use out of game knowledge, just keep quiet and let the other players take the decisions.

Playing a dumb character might help if you can't restrain yourself ^^

Note that Curse of Strahd, in particular, is pretty nonlinear.  The DM could easily run things in different places and different ways than you would have, as well as there being no telling how closely he'll stick to the book.  I wouldn't sweat it too hard.  if you can play in a game where another DM is using monsters and such you know the stats for without being obnoxious about it, you'll be fine.  Metagaming is an overrated issue anyways, if you're enjoying the game who cares whether there's a little meta slippage or not?

What I try to do;

1) Keep my knowledge separate from my PC's knowledge.

2) Keep notes on what my PC knows so it is easy(er) to remember what they know and do not know. 

3) Tell GM I have read and or played the module before and ask what they would like and not like me to do and or how to play my PC. Some times a GM likes a player and their PC to have more knowledge and then can use them to help the group with plot points and important things they might miss. Usually (in the past) I send messages over email and or make handouts with info for the player and or players in question.

4) Ask the GM if they want me to tell the group I have played the module before and explain what my limitations are and or might be when I play. Why? The other players may find out and be put off and or decide they can read the module since someone else has.    

When I'm in this situation and it's stuff I just can't forget, I tend to play very conservatively and only intervene if the party is really spinning its wheels. If your character is dumb, oblivious, or facetious you still get plenty of roleplay opportunities that don't need to be attached to moving the plot forward.

The big one you've already covered, the GM knows you've read the adventure.  Because of that, they know to throw in a few curve balls.  Like Strahd is actually just a vampire spawn in nice clothes, and one of the other characters is secretly the true Lord of Barovia, or the Vistani are guerilla warriors, striking at the walled towns which are more like concentration camps than villages, used to feed Strahd's ego as well as his need for blood.  My favorite with experienced, trustworthy players (but not necessarily trustworthy characters) is to have Strahd offer one or more of the characters everything, no strings attached (at least by Strahd) on thier first night in town.  One of them gets to be a full vampire, ruler of Barovia, everything Strahd has now belongs to the PC.  Strahd then walks off into the mist, never to be seen again.  How do the other players, and the rest of Barovia react?

But really it doesn't have to be huge, just enough to throw a wrench in your meta plans.

How far did you prep the campaign? How much do you actually remember? How I'd deal with it would be different if I had an eidetic memory and read the entire book versus having a vague recollection of the first chapter.

If you don't remember much that would count as spoilers, then don't worry, just play. If you do remember spoilers but can separate your meta knowledge, then do so and just play.

If you do remember a lot of spoilers that matter and you can't avoid metagaming, then yes, play a character that wouldn't natural contribute much. One that is unintelligent, a pet or insane or something. One where you wouldn't ever expect to contribute in a manner that knowing spoilers would be a factor.

If you only know up to a certain point, you could arrange to have your character killed off (or otherwise removed from play) and have your real character, that can contribute in a wider variety of ways, be brought in.

Or you could work with the DM. Play an NPC (or several for that matter) that works with the party. Or even roleplay characters that the players interact with.

Hopefully, if your last campaign collapsed that early, it means that you know only the beginning really, and so can use one of these ideas to tide you over until you get to unfamiliar territory where you can start playing in earnest.

Are you the type of person that sees a movie and when you talk to someone that hasn’t seen it yet you say “ this movie was so cool, so tense, and you find out at the end the main character was dead all along. What a twist that was”. And your friend is like “Dude!! Spoiler alert!!”

if you are, don’t play in this campaign. If not, have at it.