I've noticed a lot of folks warning that 'roleplaying is subjective', as a potential downfall of awarding xp solely (or mostly) based on roleplay. While this is true, I think it is something that can be overcome pretty easily.
One way to do so would be to give (allow the players to select) one or more personality traits by which to judge their roleplaying. If Player A chooses 'conciliatory' as one of his personality traits, and then makes a point of going along with what others choose, then he's fulfilled his roleplaying requirement, and the group can receive a commensurate xp reward.
Another way might be to ask the players themselves, "how well do you feel that you roleplayed during this session?" They are, after all, probably the best judge of whether they played their characters or not. Again, success in this regard should contribute to a group xp reward.