So I am making a warlock character here on DnDBeyond for fun and spot the Eldritch Invocation "Gaze of Two Minds". It says you can only use it on a willing creature, which made me think "what defines a willing creature?". Like, lets say you try to use this ability or a different spell that needs a willing creature on an NPC you became close friends with or at least a non-hostile NPC, but you didn't inform them you were going to use it, would it still work or do you actually have to inform the other person of what you're going to do to have them become 'willing'?

Willing is undefined in the rules, but the definition is usually relating to the ability to consent.

If it interest you, the Devs also anwered this question on twitter;

@Max_H_Petersen Can you take a unconscious target with you using dimension door?

@JeremyECrawford Only a willing creature can travel with you via dimension door. You can't give consent when you're unconscious.

I recall that thread, about being unconscious and I recall it being said that a friendly creature would most often be classed as willing, while a hostile creature would not.  More RaI on that, but it's how I would rule it at my table.  Any ally could be taken with you, but foes would not.

But would they still need to be aware of what you're about to do? Like do you have to inform them you're going to use dimension door or do they only need to be an ally?

The rules of D&D are written in natural language, meaning unless a term has an explicitly defined rules meaning, you use the natural definition of that term. For example, the rules don't specify that walking requires putting one foot in front of the other, but that's the natural language definition of walking, ergo characters don't glide about like bugged out video game characters.

The natural language meaning of willing is having given consent, and an unconscious individual cannot give consent. Hence an unconscious creature can never be willing.

Now, before you bring in medical consent forms and the like, those are specific exceptions for specific scenarios. They do not change the general, natural language meaning of willing or consent, which is what applies to D&D.

Also I would personally steer miles clear of the implication that an unconscious creature can give consent.

Quote from Neckerman >>

But would they still need to be aware of what you're about to do? Like do you have to inform them you're going to use dimension door or do they only need to be an ally?

DM could tell, but i'd say you must know what you are consenting to in order to be considered willing to do X.

Quote from Davyd >>

The rules of D&D are written in natural language, meaning unless a term has an explicitly defined rules meaning, you use the natural definition of that term. For example, the rules don't specify that walking requires putting one foot in front of the other, but that's the natural language definition of walking, ergo characters don't glide about like bugged out video game characters.

The natural language meaning of willing is having given consent, and an unconscious individual cannot give consent. Hence an unconscious creature can never be willing.

Now, before you bring in medical consent forms and the like, those are specific exceptions for specific scenarios. They do not change the general, natural language meaning of willing or consent, which is what applies to D&D.

Also I would personally steer miles clear of the implication that an unconscious creature can give consent.

I believe the implied consent mentioned in another thread was tied to our current understanding of consent, taken from First Aid training.  This states that assisting someone, or helping them, would be agreed to, if they were conscious.  I understand the reluctance to employ implied consent, given today's delicate and volatile views on consent, but in the D&D setting, and applied to helping an ally escape, I would still allow it.  Essentially, I would rule that anything you can reasonably assume the character would consent to is acceptable.  Similar to pouring a potion into your fallen friend.  Technically they would need to agree to drink said potion, but one can safely assume a character would indeed drink a potion that was going to save their life.  Similarly, we can assume they would resist an attempt to pour poison down their throat while unconscious.  

Quote from Davyd >>

The rules of D&D are written in natural language, meaning unless a term has an explicitly defined rules meaning, you use the natural definition of that term. For example, the rules don't specify that walking requires putting one foot in front of the other, but that's the natural language definition of walking, ergo characters don't glide about like bugged out video game characters.

The natural language meaning of willing is having given consent, and an unconscious individual cannot give consent. Hence an unconscious creature can never be willing.

Now, before you bring in medical consent forms and the like, those are specific exceptions for specific scenarios. They do not change the general, natural language meaning of willing or consent, which is what applies to D&D.

Also I would personally steer miles clear of the implication that an unconscious creature can give consent.

I believe the implied consent mentioned in another thread was tied to our current understanding of consent, taken from First Aid training.  This states that assisting someone, or helping them, would be agreed to, if they were conscious.  I understand the reluctance to employ implied consent, given today's delicate and volatile views on consent, but in the D&D setting, and applied to helping an ally escape, I would still allow it.  Essentially, I would rule that anything you can reasonably assume the character would consent to is acceptable.  Similar to pouring a potion into your fallen friend.  Technically they would need to agree to drink said potion, but one can safely assume a character would indeed drink a potion that was going to save their life.  Similarly, we can assume they would resist an attempt to pour poison down their throat while unconscious.  

You don't need to be willing for someone to adminster you a potion.

Honestly, dimension door and other teleportation spells are almost the worst examples, because from a story perspective there's no reason not to allow them to be used on an unconscious creature. You're basically saying you can't get an ally making death saves to safety with those spells, or a villain can't whisk away an NPC when the party is distracted, etc.

I completely get wanting to model good behavior on the concept of consent when it comes to things like polymorph, and not wanting teleportation spells to have any effect at all on creatures that don't want to move (rather than having them make a saving throw or whatever), but the language being used in the rules isn't really adequate