I always like hearing actors talk about their experiences in terrible movies. If they're allowed to call it terrible rather than say "oh yeah, it's great" while promoting something like "Anaconda 23:Still just about Giant Killer Snakes!"
I love John Barrowman/Captain Jack. Barrowman doesn't act like a different character when he plays Jack. He just acts like he was himself, but from the distant future. And that just makes it so much fun to watch, for some reason.
I saw him in person a few times at Comicon a few years ago. He is always on and utterly charismatic, willing to hop on a signing table and sing at a moment's notice.
I'd say that for Captain Jack he turns down the wattage just a little bit.