This is how I got my crew to Roleplay just last Saturday.
I picked my easiest going player (since she would be out for a while), then I had a local rogues guild feed one of the players 2 poisons. One to make her Nauseated and the second to make her pass out after one minute. She ran out of the tavern to retch on the dirt instead of the tavern floor, then promptly passed out.
Rogues picked up her unconscious body and carried her off. The other players had to use RP to get information to find the location of the rogues guild and their companion.
If they don't RP, make 'em MWAH HA HA HA