Lancelot is really just wandering in for a cup of coffee in between his morning drills and his lunch with Susan -- he has a few other things to accomplish, and coffee would help. However, upon seeing that there is ... some kind of set-up, he comes to an abrupt halt, just inside the doorway, glancing around as if trying to figure out what's happened. All of the color, the music, has the effect of making him feel he's stumbled into a party that he didn't even know would take place.