This is probably really obvious but my mind is blank this morning. It's more likely to be a film than a TV show, and is guess at some point within the last 25 years (1990 onwards).

There's two suspects and they are each placed in police interrogation rooms next to each other. The detective goes back and forth questioning each of them. There's a senior cop watching all this, who asks his colleague "What is he doing?"
Eventually the detective coerces one of the suspects into saying something that vaguely implicates the other when taken out of context. As the first suspect says this stuff the detective holds down a button that allows the second suspect in the other room to hear his friend speaking. At this point the detective goes into the second room and the suspect confesses.

Any ideas?