Not sure if Martin Lawrence was mentioned in this thread, but I've seen him play around and shadow box and you could tell he knows what's he's doing. Everything from the stance to punch form, head movement, and defense.
In one of his episodes in the 90's he was fake fighting against Tommy Hearns. If you've seen the episode where Martin is playing defense against Hearns while being against the ropes, you can tell he's boxed and knows what he's doing.
I'd be interested to see him in action when he was younger.