There's a part in my WIP where the heroes stop for a minute to listen to a standup comic, and I just realized something...I have no idea how to write standup comedy. Here's what I've got so far. How would you improve it? "Lemme ask you something, folks," a loud voice came from the front of the room. I looked up just as a man walked onto the stage, microphone in hand. "Have you ever seen a horror movie? You know, those old fashioned slasher flicks. Have you ever thought about how unrealistic those movies are?" [...] The comedian went on, "I would love if somebody made a slasher film that played out how it would in real life. Imagine this: the killer is breaking down his victim's front door, a butcher knife in his hand. He gets inside, and — BANG! The homeowner nails him between the eyes with a glock. Sorry, pal! Next time try look up your local gun laws before you go on a rampage."
It sounds pretty good to me, like a real standup routine. The only thing that seems a little strange is that the comedian starts talking before he walks out onstage. I usually see stand-up in clubs done with a mic on a stand, not one they carry around, though they might pull it out of the stand later and carry it. Usually though it seems to start on the stand. But I'm sure it can be done the way you described too. If I were going to write a stand-up scene like this I think I'd want to know if it's a good routine, a bad one or what, and make sure to incorporate that into the writing. You'd write very differently about a good act and a bad one. That's something I think readers would want to know. Or maybe rather whether the MC thinks it's good or not.
Stand-up comedians, particularly at the start of their careers, don't know what's a good routine or what's a bad one. And the good ones are all over the map stylistically. They range from Robin William's manic flights of fancy to Steve Wright walking out despondently, standing in front of the audience in utter dejection, and then saying, "I don't always feel this happy." But these acts are the result of years of honing their skills, knowing their audiences, and figuring out the right timing for their schticks. But most novice comedians rely on comments about girlfriends or boyfriends, or politics, or whatever they think the audience can relate to. As for the comedian's dialog in the above story, I'd say that that joke was over the moment that Indiana Jones confronted the sword-wielding bad guy, pulled out a gun, and shot him dead.