- Disposed or inclined; prepared: I am willing to overlook your mistakes.
- Acting or ready to act gladly; eagerly compliant: a willing worker.
- Done, given, or accepted voluntarily or ungrudgingly.
Definition 1 is effectively motiveless, in this sense "willing" means that you are prepared to do something; your motivation for doing so is irrelevant.
Definitions 2 & 3 expand on definition 1 to incorporate a motive, you are "willing" if you are "acting gladly", "eagerly", "voluntary" or "ungrudgingly".
My personal preference for a "willing creature" is to adopt definition 1 - if they consent to the spell then their motives are irrelevant. The other definitions are problematic as the motive must always be a matter of degree.
For example, if you were to agree in a negotiation to do something then you are clearly willing by definition 1 but you may or may not be willing by definition 2 and 3. When you ride a roller coaster in spite of your fears, are you willing? Or are you unwilling because you harbour doubts or are submitting to peer pressure?
Are you a "willing worker" because you derive deep satisfaction from the job you do or because you have a mortgage to pay? If the latter, are you in fact willing for definition 2?
What is willing
Prima facie, for all definitions, if you ask, and they say "yes" then when you cast the spell, it will work.
Further, the target of a "willing" spell must knowingly consent to be a target (PHB p.201):
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.
If you were to simply cast the spell on a creature it would not know that it was the target of a spell. In general, it would be fair to assume that a creature would be unwilling as a default state as being willing would expose them to anything any spell caster wanted to do to them.
Making the unwilling willing
So, if they are unwilling, what can make them willing?
- Persuasion you can use your golden tongue to convince them. This is a negotiated agreement so it works for definition 1 and probably 2 & 3.
- Deception you can tell them you are casting spell X and then cast spell Y. It would be a DM call if willing is a general condition or specific to spell X. My personal feeling is that it should be general.
- Intimidation if you really scared the c**p out of them they would probably be willing to do what you want by definition 1. This leaves you more open to the "bait and switch"; they agree to teleport away with you and go "unwilling" at the last second. This would not make them willing under definitions 2 & 3.
Charm makes it easier to do any or all of the above but would not of itself be sufficient:
The charmer has advantage on any ability check to interact socially with the creature.
Suggestion seems perfectly feasible (for all definitions), if you can word the suggestion to make subjecting themselves to the spell seem reasonable, it says:
You suggest a course of activity (limited to a sentence
or two) and magically influence a creature you can
see within range that can hear and understand you.
Creatures that can’t be charmed are immune to this
effect. The suggestion must be worded in such a manner
as to make the course of action sound reasonable.
Dominate again problematical (under definition 2 & 3). The command "be willing for this spell" may be sufficient to overcome this but the DM would need to decide.
You can use this telepathic link to issue
commands to the creature while you are conscious (no
action required), which it does its best to obey.
Personally, I think "which it does its best to obey" makes it willing for definition 1 and I think that is the threshold but others may disagree.
I would think that using this part of the Dominate spells would make the creature willing:
You can use your action to take total and precise
control of the target. Until the end of your next turn, the
creature takes only the actions you choose, and doesn’t
do anything that you don’t allow it to do.
Conditions What are the effects of various conditions on a creatures willingness?
I believe the following would make no difference: Blinded, Deafened, Frightened, Grappled, Incapacitated, Invisible (though it may make them an invalid target; Hidden would be similar although not a condition), Paralysed, Exhausted, Poisoned, Prone, Restrained and Stunned.
Charmed has been dealt with above.
So that leaves Petrified, Unconscious and Dead (technically not a condition but close enough). For the first 2 (and presumably dead) the creature "is unaware of its surroundings" so, can it be willing or unwilling or even treated as an object rather than a creature? Clearly a DMs call but I think that it should depend on the particular spell. I will only give 1 example but there are some imponderables here.
Consider Teleporting with an unconscious creature; if the creature were an ally of the caster then they would (but for their unconsciousness) be willing to be teleported (e.g. away from the ravening dragon), conversely, an enemy would be unwilling (e.g. to prison). To my mind it makes more sense to treat them as an object i.e. their willingness or otherwise is irrelevant and they count against the weight limit rather than the creature limit. However, it is equally valid to say all unconscious creatures are willing or unwilling or as they would be but for their unconsciousness. Your call.