Well, this is how I forced my PCs to RP in my Kingdoms of Kalamar game. It takes place in Geanavue, so they already knew that the LAW would come down on them if they became hack'n slash bunnies.
I started them out meeting at a friend of their respective families for the Festival of the Dragon, a major holiday on par with Mardi Gras. They had a few days to relax and RP before the festival. I kept describing to them the preparations of the feast, the positive energy in the air, and all of the colors, sounds and smells that assailed their senses. Soon they were amped for the Festival. The day of the festival many DMs would have some dark Mardi Gras type plot pop up. But I didnt. I let the RP partying and interacting with NPCs. They learned the fun way that not every NPC is out to get them. Instead they made many new friends and allies.
It was only a few days (in game. Out of game it was 3 sessions later) that the plot was introduced. But even then, they were so used to roleplaying that they preferred roleplaying through the adventure rather than hacking through it.
You have to subtly condition your PCs to know that not every NPC has some dark secret or is out to get them. In addition, highly descriptive settings sets the mood for them and they begin to envision the world around them. They will interact in a realistic way.
If your players dont care about RPing after doing this, and still sigh when you get descriptive and look for things to bash, you need to either get used to running dungeon crawls or get another group of players.
::shrugs::
-=grim=-
