I actually run a fairly serious game, but my players don't really take it seriously. Almost anything my NPCs say, the players will find a way to make fun of it. I can rarely seperate IC and OOC comments as 95% of speaking is OOC. So when I say "The menacing figure steps up to you and say 'How dare you enter my temple? What are you doing here?'" the players are likely to answer "We're here to kill everyone in here and take all of your stuff." They don't show the slightest bit of fear, awe, hesitance, or anything. They just assume that the guy is obviously low enough level to beat so they need to kill it.
They also turned a possibly perilous encounter with a mind flayer into a joke. The mind flayer managed to use charm monster on one of the silliest players. He told him "tell your friends that you killed me and get them to leave".
So the players turned around and said "My friend, the Mind Flayer said to tell you guys that I killed him and you are supposed to leave." Then when I gave him a look like he was an idiot he said "What? My character has an intelligence of 6. He doesn't know any better."
This player is such that no matter how serious my game was, he'd be like this. So, I put up with it because he's my friend.