RP doesn't happen spontaneously. Well, sometimes it does. But, especially when your group's not used to it, you need to make it happen.
Set up a situation that can't (or, at least, really, really shouldn't) be solved through combat. For example, the PCs need to get information from an NPC, or need to persuade an NPC.
Roleplay the NPC yourself. Be over the top. Be ridiculous and goofy. If you don't feel like you're making an ass out of yourself, then you're not doing it right. The key is to get the players laughing and involved with the character himself.
If (or more likely when) the players say something like, "Ok, can I roll Diplomacy?", tell them "Yes, but if you do it in character I'll give you a +5 bonus on the roll." Don't force them to... but strongly encourage them to.