I don't know. He may not know anything about the issue, but my impression was he was asking her "unexpected" questions - things she didn't have a prepared answer for.
She would love to get asked something she knew and could try to sound intelligent about. Instead she stuttered and stammered. And he asked her flat out was Bush a liar. Her hatred dripped on the screen.
His job is to expose her. I think he did a good job.
He could have taken two approaches in the second segment. One would be to go after the technical issues. She has already shown she will shift away from that argument with the one-in-a-quadrillion possibility that the memos COULD have been typed in proportional fonts by a Lt. Colonel in TANG for his own personal files.
But O'Reilly took a different slant. He said there should be a standard in this situation of proof beyond a reasonable doubt. And then he asked her if that standard had been met.
And Mapes said something very revealing - she said basically in her mind that they had been.
And that is the mantra of the modern liberal - their beliefs are all the proof that is needed, and all the other evidence to the contrary is just right-wing talking points. 200,000 dead in Iraq? I believe it so it's so. Bush stole Ohio in 2004? I believe it so it's so. Bush lied to get us into war? [repeat ad naseum].
So in one way O'Reilly missed the chance to trip up Mapes on the details. But in another way he revealed the true underlying pathology at play here.