In general, I like social situations, like combat, to be influenced by more than one roll, and by more than 1 PC. What I like to do is have the PCs enter into a situation - for example, they decide that they need to convince the guard on the castle gate to let them in without the proper authorizations.
I have the main PC roll to see what sort of initial impression he makes. For example, he might roll a 17 (+5 for his skill) on diplomacy as he approaches. Thus, he notices the guard is smaller than average, has thick-soled boots on, and stands very tall as the barbarian PC approaches. Plainly, I say, he doesn't appreciate feeling short. Thus, the Barbarian slouches a little, and makes a point of speaking in a way that shows respect for the guard's authority. The PC either roleplays or tells me (whichever HE prefers) what he says, and I roll for the guard's response. He rolls very low on his spot roll, and high on his attitude roll, so he fails to observe any flaws in the barbarian's story, and laps up the "I'm a big guy" vibe he's getting. However, he does have a job to do, so he speaks to each of the PCs in turn, checking for their reliability. Each other PC then responds, with a bluff or diplomacy roll, whichever they prefer (either to hide their attitude, or to blend in inconspicuously).
Because of the high roll of the barbarian earlier, I give an easy roll (perhaps a DC 10) on this roll to the rest of the PCs. The guard is relaxed and inclined to let them in. Had the earlier roll been bad, and the barbarian made the guard uncomfortable and prone to take insult, the DC might be a 15 or even higher...
In situations like this, good roleplay is PLAYING out what the dice and the character's skills have already predicated. Sometimes it's more fun figuring out WHAT the stupid dwarf cleric managed to say that caused the guard to spit in his face and try to arrest him on the spot...Bad roleplay just means taking the dice at face value and not trying to "work out the scene". Nobody is penalized, they just don't have as much fun.