My players have never been big on roleplaying and that's fine. But I think there is a bare minimum required to separate the game from "Kill monsters, Get money." That's a bit of an inflammatory statement, there's nothing wrong with kicking down doors, killing orcs and taking their pies. For me it's one of the most fun parts of the game. However, the game is more than that, and aside from 1, maybe 2 players most of them don't try to branch out past that giving a chilling effect towards newer players. That's beginning to change and I'm very happy as a DM to design something more than an exciting combat.
Here's what I've been doing. In this particular campaign I had each player play through an opening vignette that established their motivations and the reasons for how they ended up in Hommlet. This was the first session for the player's present. During these vignettes I took notes on the players actions and the mindset behind them. I wrote down different character traits and jotted down a quick bit of reference for what it was referring to. At the end of the session I had them write down these character traits and explained to them that these were based on how their character acted and were useful in developing a personality.
(I should clarify, about half my players have never developed a character personality separate from their own, I'm lucky that they even choose a name sometimes hah!)
I let them know that when they continued to exemplify these traits during play they would be rewarded with experience for each major occurrence. Furthermore that they could earn new traits through roleplaying. This creates a positive feedback loop, that by playing to a character personality that their actions created they would be rewarded with experience. The more they do it the more experience they earn. Since then, my one player who frequently falls into the trap of chaotic=sociopathic, has been my best roleplayer. He pays close attention to his actions and does his best to make sure his actions coincide with his earlier displayed traits and has earned a few more as well. As an example he recently added 'benevolent' to his traits as he took it upon himself to become foodclaus after being rewarded with a surplus of crop rather than traditional coin.
Here are a few traits (after the break) and the situation where they displayed them:
Curiosity - "Gee I wonder what this magicians staff does."; "I bet that bear is hoarding something."
Lucky - Numerous lucky rolls that saved him from harm or caused him to find an important item. This was later subverted and turned into Unlucky after a series of critical failures that culminated in a Scroll of Raise Dead backfiring.
Reckless - Found some staves clutched by skeletons wearing decomposing robes. Rather than taking a moment to examine he immediately ripped it from their cold dead grip and read aloud the command word he discovered. There is also the issue of the bear.
Loyal - Despite being grievously injured he charged headlong into a rampaging bear to save his companion.
Trusting - His companion offered him a drink of strong spirits after surviving the bear. Unfortunately the drink was drugged so that his companion could steal the treasure map and strange icon they had discovered.
Good-Natured: He found a ring with an inscription indicating it was a gift from father to son for becoming ordained. When finding the priest in Hommlet he willingly gave it up without want or need for reward.
Duplicitous - Spent a long time impersonating a worker, then a foreman and a magicians apprentice playing each of them off of each other.
Non-confrontational - Did his best to avoid the use of violence
Pity-player - Appeared to be weaker or helpless to avoid suspicion or to play someone
Observed from afar - Spent most of his time observing before interacting with NPCs
Advantageous - Took advantage of every opportunity that came his way, regardless of who it influenced
Powercrazy - Gathered enough incriminating evidence on his employers to turn them in and free him of his contract. Instead chose to be a partner with them with the goal of eventually backstabbing them and taking control of the organization himself.
Sharking - Preying on the weak or unintelligent.
Easily Deflated - The innkeeper offered him a free room (the swindlers actually) if you could get evidence on the swindler. After losing some money he made a few smart plays but failed in their execution and quickly gave up. Fisticuffs were later used to resolve the problem.