If you want people to roleplay do it often youself and see kill who don't joins in(i usually do)
Very few players actually understand roleplay as thinking about a background for your character and act in-character all the time. For some, roleplaying means making up some vague story to put behind a faction name they're using to pwn enemies. For some others, it's simply standing around in a town, talking nicely to people instead of shooting them.