One thing I have found that will start some roleplay at table with reluctant roleplayers is to "roleplay out" the combat as much as I can. With every action or blow, I describe in detail what the monsters are doing. In other words:
"The bugbear who looks the biggest and meanest advances on you, his mace in hand. He growls as he moves in on Granthar. The other two snatch up javelins from the floor, and begin shifting into position to get a clear line on Bruin." Then I ask each PC, in initiative order, to tell me what they're doing.
If they give a plain answer, such as "I attack the big bugbear", I'll respond "okay, you swing aggressively with your greatsword. The bugbear looks surprised at your attack. Roll to hit!"
If they respond with a little more detail, I will give positive feedback, and possibly even a small bonus to hit if I feel they're being particularly "good" in their play. Once they're roleplaying in combat, they're far more likely to turn to each other and say "Good job, Granthar!" than "Kyle, you hit!"
From there, the roleplay DOES gradually increase, in my experience.
