I have never found rewards (XP, action points, Karma, etc) a good incentive to roleplay. I have found DM/GM encouragement and putting the work into working directly with the player is the best motivation (side stories that don't involve the plot directly). Maybe a character's sister shows up. Maybe a character wants to open a bar. Maybe a love interest. Maybe a rubbery villain (one that always seems to get away and irritates one particular character, perhaps talking with a silly french accent, but never is actually any danger to the character or the other characters in the party, just irritating). Every player has something they like that doesn't involve rolling dice and killing things. Find what those things are and make them something special and interesting only for that particular player. Just don't devote a large chunk of time to these in game since they focus only on one particular player, leaving the rest sitting around eating pizza.
Example
In my usual group we spend most of the time running PF APs, which doesn't always leave a lot of time for roleplaying necessarily. We were playing Cauldron and my Cleric/Wizard decided that the town needed a temple to Bocoob. It had nothing to do with the AP but it allowed me to have some roleplaying fun. I bought a building, turned it into a temple (which was really just a library). I spent my treasure helping the residents. This eventually became part of my roleplaying inside the main story of the AP. I had to think how my character's actions would effect my followers. I couldn't just kill the bad guy disguised as a town leader, I had to expose him first so that my followers wouldn't think I was out to take over the town. Basically my "side story" influenced my characters reactions and encouraged roleplaying that wouldn't have normally happened. We didn't spend much time on it "in game," most of it was done through emails or during the "set up" time before the game started, or when we didn't have enough players that night to run the AP, we'd still get together and play around with the side stories.
When I run games I try to create NPCs that work with the particular player's preferences, rather than necessarily the campaign's story. If a player likes to play the drunken dwarf, give him a drinking companion. If the player is the type of guy/gal that likes a little drama, maybe have the companion hit by a runaway wagon one night while the characters are off saving the world. He'll return and find him in need of a raise dead or need the character to sneak him in some beer to the hospital without the nurses finding out. Maybe he dies and players goes to the bar to drink away his sorrows when the companion's wife shows up and tells him how now she doesn't know how they'll survive without his income from working in the mines. The next thing you know, the player is putting aside money for the widow, maybe checking in on her when he's in town. Maybe he buys the bar and has the widow run it. The point though is to keep it to a minimum at the table, just enough to remind him/her that there's life beyond the adventure.
Anyway, that's the type of roleplaying I like in my games, especially when there's not a lot of time at the table for it. We're all busy people and I know I like to unwind with a good "kill em all" when I play. But I also enjoy the side stories that make my character 3 dimensional. I don't need rewards, only a DM willing to work with me on developing the side story.
