'We tend to believe people when it costs them something to say whatever it is they have to say. It almost takes the form of a mathematical equation: the more it means to a character, the more difficult it is to -- Robert Towne
Make sure you've got the proper side of the triangle on stage. If the conflict's between B and C, you won't get anywhere with a scene between B and X, if old X is just curious, helpful, thoughtful, and sympathetic. And if X basically won't face any real changes if dear C jumps off the bridge or not.
When you stumble into this situation, there's a simple
Reshape the scene so it's between the two folks who have a real stake in the outcome: C and B. Send X to the bathroom, or to answer the phone, or to Pago Pago.
When the wrong side of this sort of triangle has the stage to itself, tension drains out of the dialogue and often after a page or two the whole business comes to a halt -- not even a screeching one at that. Great listeners may be nice to know in real life, but they'll make your life as a playwright miserable if you let them sneak into your play.
http://www.vcu.edu/arts/playwriting/