I have been in the same situation. Me as the DM hoping for a deep roleplaying experience, the players just wanting to kick some ass, drink some beer and have a few laughs. However I have managed to get my players to 'come out of their shells' a bit. Here are a few of the tactics I found successful.
* Start off with humorous adventures. I believe everyone is heard of the saying 'Throw a frog in boiling water and he jumps out; throw a frog in cold water and bring it to boil and he happily boils to death'. People tend to have an easier time getting into character if it is done in a humorous way. Throwing deep character-driven plots at players not accustomed to roleplaying and expecting them to seriously interact with the characters in a meaningful way is too much. Especially in a social environment such as a tabletop RPG. However, if you have a couple of over-the-top adventures, laden with memorable NPCs that have punny names and cliche mannerisms, the players might be more willing to have some fun with them. At first I would keep it clear and memorable - none of that 'Arashorm from the town of Shillelaighfast' stuff; instead you can go for such lameness as Norris the Woodchuck from Detroit, the small fishing village. Over the top, caricature characters are much easier to intreact with in-character than dark, brooding anti-heroes. Once you get them started on roleplaying with humour it just takes a small nudge for them to find themselves roleplaying in a deeper, more 'realistic' sense.
* Introduce roleplaying elements into combat. When the fighter in your group kills an enemy ask how it happened. With some quick thinking you can reward him for such small things: 'Um... I guess I slice him in two from the waist' - 'Awesome, the speed and force of such a blow sets you up nicely to attack the next adjacent enemy. You have +1 to your next attack'. Don't worry about unbalancing the game. In my book, roleplaying and houseruling/fudging go hand-in-hand. The rules are there to provide a framework and make the game fun, not to keep your imagination tied up.
* Know your world and know your characters. For me the prime rule in playing as a DM (as in any improvisational art) is to say yes. In order to be able to say 'yes' you yourself also need to be ready to roleplay. It doesn't bode well for the group's roleplaying exploits, if you allow yourself to be caught off guard. Maybe the PC who is currently speaking isn't interested in hearing what your NPC has to say. Maybe he wants to ask about a rumor they might have heard and pursue a sidequest. If your NPCs always say 'No, I don't know nothing about that, sorry bro' the players might take that as a pointer to keep on the rails and not bother with roleplaying.
* This one is a bit of a touch-and-go proposition. Look at the character sheets or tell the players to introduce their characters and their backgrounds to you, 'where they're coming from' so to say. Try to introduce situations that allows their character to surface. If they don't follow the cue, try to nudge them a bit, 'Doesn't Paul the Fighter who was impoverished for half his life have to say anything about executing these thieves?'. Doing this you must use a certain amount of finesse - the players might be insulted or angered by you trying to play their characters for them. You shouldn't force them, a slight reminder is enough.
* Don't press anything. If in the following situation your player responds with 'Well, Yeah, I guess we should um tell the guards to let them go or something' - that is good enough to start. Reward them with their favourite part of the game - for many people it is the excitement of combat or exploration; maybe the thieves they have to subdue the guards to save the thieves or maybe the thieves give them directions to a secret stash of stolen magic items that has been overrun by monsters. You can roleplay back, but if they don't respond, just go with it, happy that the PCs personality has in some way influenced the world. It can and will get better once they get used to it.
In regards to the problems with roleplaying evil characters - keep in mind that evil doesn't mean stupid, just as good doesn't mean lawful stupid. It is just an alignment, being evil doesn't mean that they aren't functional (non)human beings. They might usually be called just as neutral as your average Joe from the market square, if it weren't for the goal they are working towards. Nearly nobody does evil because they wish to do evil. Evil characters usually believe that they are doing good (for themselves or even for the whole world), just that they are misunderstood. An exceedingly small percentage of people 'just want to watch the world burn' - in my games, none of them are PCs. I don't believe that you can have a functioning game with characters like that in the leading role. If your players need inspiration on evil functional human beings I would point to such TV series that have 'evil' characters in leading or important supporting roles. Deadwood and Boardwalk Empire come to mind.
Keep working on it, things will certainly pick up! I speak from experience.
