That sounds incredibly odd ... if there will be any "change talking character" option (and i also hope that there will be) personaly i hope it will be without any interaction, simply one person stopped talking and another started.
In this case it would be not your character who see that is going to negotiate, and its not entirely his area of expertise ... but Gale, or Wyll see that, and simply choose to act, spontaneously and without prompting ... it also kinda seem to me to give a better impresion of characters, since they like this are (in Roleplay perspective) acting by themselves, and finaly dont need curent "main character" to tell them what to do.