I've done radio interviews, and it's not hard for me. I've never had trouble with public speaking. But you have to keep focused, exclude the distractions, or you'll get caught in the moment and suddenly everything evaporates. You lose your train of thought, you can't follow the conversation, you sweat profusely ... It's like a comedian who drops a bomb, then goes to pieces trying to get the act back on track.
I would think that focus would be a cinch for a seasoned performer like Chuck Norris. But filling in in a strange position, with the high profile Sean Hannity has, he may have dropped his guard for a second and bam! lost it.
I feel for the guy. He was apparently out of his element.
The fact that he tried makes me wonder where he is heading.