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.
I suggest the following: make it a part of the narrative. Make the character get hurt. Inflict some damage on him, use insanity rules if your game has them. Get into the character's psyche. Tell the player that if he doesn't comply, he'll get hurt. A lot.