Say somebody says "I'm going to use a spell to charm you, and make you walk around the room," for a bet or something. You know it's coming, you know your goals, but what effect does this have on saving throws? What does the charm do to your thought process? Might you have advantage on the saving throw if you're mentally prepared? Or does it simply mean that while charmed you still might not do as they say? How does this apply to more powerful charms?
The DM could decide to give the forewarned target advantage on their saving throw against Charm Person
Charm Person says (emphasis added):
[The target] must make a Wisdom saving throw, and does so with advantage if you or your companions are fighting it.
In the situation you have presented, there is no combat currently happening, so the clause highlighted in bold does not apply. However, this clause establishes the precedent that a target's antagonistic disposition toward the caster can make them harder to charm. So, depending on the nature of the bet, the DM could choose to give the target advantage on their save. This probably wouldn't make sense for a good-natured bet between friends, but would make a lot more sense if the bet was between bitter rivals for high stakes. In general, the DM is encouraged to grant advantage or disadvantage based on the circumstances around the roll, even when no specific rule specifies that the roll should get advantage or disadvantage, as is the case in this question. From DMG Chapter 8:
In other cases, you [the DM] decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result.
However, the DM is not required to modify the roll in any way. They could stick to the literal interpretation of the spell and decide that a target's hostility only grants advantage if it is strong enough for the target to literally be fighting you.
Other spells may behave differently
Even if the DM rules that this situation would give the target advantage against Charm Person, the same ruling need not apply to all charm-based spells. For example, Crown of Madness imposes the Charmed condition but doesn't include the clause about targets having advantage on their saving throw if you're currently fighting them, so it wouldn't make sense for them to have advantage as a result of mere seething animosity either.