If you have a powerful NPC with the adventurers - one who outranks them - it makes sense for them to handle any sort of tricky negotiations. Of course as a GM, you really want the players to have to deal with the situation, rather than simply letting the NPC do all the talking. So you need to establish somehow that the NPC needs help, because things are going badly.

And when you do this, again by applying logic to the situation, the NPC should appeal to the PC most likely to be able to help.

Transcript

GM: Queen Amidala steps forward and—
Qui-Gon: Who?
GM: The Queen. You know. The... Queen.
Qui-Gon: We sold her to get weapons, didn't we?
Queen Amidala: Oh great Boss Nass, the humans of Naboo need your help to defeat the evil Trade Federation. Will you help Us?
Boss Nass: Nosa.
{beat}
R2-D2: We're screwed.
[SFX]: < doosquee bebebloop >
Obi-Wan: Right, it's up to us then.
Qui-Gon: Oh, that's right... it was her clothes we sold.
R2-D2: Don't make me repeat myself.
[SFX]: < doosquee bebebloop >
Queen Amidala: Help Us, Jar Jar Binks. You're Our only hope.
R2-D2: Only Hope!!! <chug>