Question: How should a GM role-play a torture scene for a PC?

In one of my games, one of my PCs will be captured by the enemy who wishes to get information from him, using... less than ethical methods.

Leaving whether or not the information is divulged to a dice throw seems a waste, but on the other hand, it's hard to force a player to give up info if he doesn't want to.

I'm playing an L5R campaign, with the L5R system.

Answer: 
There is a third option, other than pure roleplaying and rolling dice, which is to offer a choice.

For example, here's a difficult choice. Either the character gives up the information or suffers a random amount of damage. Make it clear that, if the damage dice roll high enough to kill the character, the character will die.

So, then, the player chooses between giving the information or a chance the character will die.

IF you prefer something more narrative, try this. Either the character gives up the information or someone close to the character dies. 

Alternatively: either the character gives up the information or a prize piece of a equipment is destroyed. Either the character gives up information or they lose an arm. Either the character gives up information or *another* PC takes damage. There are lots of possible choices: pick one that will engage your group.
***Torture*** is a very sensitive topic, and the first thing you need to do is have a conversation out of game about how the players feel about such a scenario.

People have history, and issues, and ultimately *Torture* is mental, if it works at all. Unless you want to cause a mutiny, you need to understand how the players think. And maybe best to have one-on-one conversations with individual players to find out their thoughts. My experience with Larp is that when you have sensitive issues, you need to get permission, or just don't go there. *(And that experience translates to tabletop, and even online gaming).*

Some people have no problems, as this is just a game, but other people may not like the loss of control (even if the control is only their game avatar). A player may have defined their character as someone who would react in a certain way, and 'forcing' their character to react differently violates the essential control they need in their fantasy. I have role-played both sides of the torture thing (serious Larp'er), screaming, dramatics, but I also respect other people and their needs.

*(I apologize for drawing from Larp experience for this response, but I think the advice is pertinent to tabletop and computer/online games as well, as the following added story illustrates.)*

Many years ago, I was playing in an AD&D campaign with other college students, and we had an encounter that went badly. A torture sequence was role-played, and a young lady was very offended. She and I had a long discussion where she related her feelings, and I came away from that wiser, and more sensitive.