Regarding Val Kilmer, he was being treated for throat cancer I believe so I wasn't even sure he'd be in the movie in the first place. He looks and sounds so different and at this point, I just hope he's doing well let alone be in this movie.
Regarding Tom Cruise and stunts... well for the last mission impossible movie, the guy did all the car driving, helicopter flying, bike riding, sky diving and all the ground stunts. He took lessons for whatever he didn't already know, the guy is a machine.