This is where the "narrative" part comes into play. Make the player narrate their attempt at being charming and take that into account when determining difficulty, or dice pool,or if they wow you with their idea just outright deciding automatic success (or failure if they wow you with face palm).
I have never agreed with this approach as it penalizes players that are not able to quickly think on their feet and you are actually using the PLAYER'S Charm (or Negotiation et al) and not the CHARACTER'S Skills to make the judgement on.
For example, you would not recommend that a player punch you (or hit you with an axe) to modify how well their brawl or melee attack would do.
Only because my combat oriented players are beefy dudes who can hold their own in a fight!
