Basically in my story a cop is forced to testify in court against the defendant who is a member of a gang. The cop however, is also a part of the gang, working as a mole for them in the police. The cop didn't plan to be testifying against one of the members of a gang, in which he is a part of. It just worked out that way. The cop has to play along and testify though, cause if he refuses, or tells certain lies, than the other honest cops will know he is lying and has something to hide. So the defense lawyer has to cross-examine the mole cop on the stand. The lawyer knows that the cop is a mole, cause the defendant told him. But no one else knows. So the lawyer has to discredit the cop as a witness to help his client. Would he tell the court that the client told him that the cop was a mole working for the gang? The cop would of course refute this to defend himself, but would a lawyer do that in an attempt to create some sort of discrediting of the witness with reasonable doubt?
I'm in a bit of a rush, so this will be briefer than I would like. I assume you're talking about the American system, but I know what I'd be doing in an English court. A good barrister never asks a question he doesn't know the answer to. And on cross-examination he will want to be the man dominating proceedings. Witnesses rarely - very rarely - cave, so putting his defence case forward is the main thing. Witnesses tend to be slippery. They deny everything on cross. They never break down. As a police officer, the witness would be experienced at being in the box. He'll also know how to command the court. A barrister would need to think about how to approach it and where to find leverage. Best way is to build 'em up to knock 'em down. "Mr Jones, you say that you saw Mr Smith enter the premises by the back window." "Aye." "You recognised him because he has an unusual tattoo on the back of his neck." "That's right." "This tattoo is of a whale fornicating with a beach ball." "Yeah." "You've known Mr Smith for about six years, haven't you?" "Yeah." "So you know him and his unusual tattoos well." "That's right." "But you were almost two hundred yards away from the break-in, weren't you?" "It was a distance." "It was also two o'clock in the morning." "So?" "The best lighting was street lighting from the next street over." "And? I know I saw Mr Smith." "You couldn't see his tattoo at that distance, could you?" "I saw it, I know I saw it!" When I'm back from my weekend away, I'll try to explain more thoroughly. The above is a damn good cross-examination, though. What the witness says is discredited. Focus falls on the lawyer and his case. I see reasonable doubt. That's the sort of thing your lawyer will have to do.