Going through the spells, I see the phrase "Willing creature" used to determine who you can cast some spells on. I don't ever see a proper definition of what a willing creature is, but I would assume that it would go something like if you asked them "Can I perform this spell on you" they would answer yes.

But can you ever make someone who might have said "No" say yes? Such as using a Suggestion spell? Are there any spells or effects that can make an unwilling creature willing?

Going through the spells, I see the phrase "Willing creature" used to determine who you can cast some spells on. I don't ever see a proper definition of what a willing creature is, but I would assume that it would go something like if you asked them "Can I perform this spell on you" they would answer yes. Is there a more formal definition of what makes a target willing in 5e?

What are the ways I could make an otherwise unwilling creature willing? Suggestion spells, for instance, should probably work but what other options do I have?