I think the reason those four guys were cast was specifically because they don't look like gangster types. It would've been a much less interesting scene if they would've pulled up to an Italian restaurant and sat down with four fat Italian gambinos. That's been done a thousand times. Part of what makes Pulp Fiction so clever is that it is a gangster movie but it's an atypical gangster movie. The "gangsters" stand around talking about cheeseburgers, milkshakes, and other nonsense instead of talking about frank sinatra and tony bennet or which bitch they're gonna bang next. And as for Marvin, it's been awhile since I've seen the movie but i'm pretty sure when Jules and Vincent are getting the guns out of their trunk they discuss how many guys are inside the apartment. And they make a reference to "our guy", something like "is that including our guy?" Referring to Marvin. So yes, Marvin was connected to Marcellus Wallace in some way or another. I would guess he was scared b/c he just saw his associates blown to pieces and he was probably a small time guy who wasn't used to that sort of violence. Just my opinions though.

I think MurraySiskind has it right. Also, the "cheeseburger" scene was sitting in the car at the begining, and the "milk shake scene" was when Vincent and Mia were at the bar.

Umm, the scene in the car is the "Royale with cheese" scene. The "cheeseburger" scene is when Jules tries Brett's Big Kahuna cheeseburger.

It's doubtful that Wallace had Vincent at Butch's apartment to punish him, as Wallace was there himself.

It's Wallace's gun that Butch uses to kill Vincent. He went out to get coffee and breakfast while Vincent stayed waiting on Butch. Wallace was there because Jules quit, and there apparently wasn't anyone else available for back-up.

And Wallace -- it's probably been a while since he's done this, so he got sloppy and left his gun behind. Jules wouldn't have done that, you know. And Butch would've been dead.

And, as for the four guys in the apartment, we don't know what was in the briefcase or why they had it. So that part of the story wasn't told. It's probably better that way, to leave it a mystery, and the let the viewer's mind fill in the blanks.

Marvin. He might've actually been someone sent by Wallace to infiltrate the other three guys' conspiracy, and it seems likely, as it's implied that Jules knows him, but isn't apparently on very friendly terms with him. Notice how he lets Vincent bully and threaten him without telling him to back off.

My guess is that Marvin was already friends with the other three, found himself getting in over his head, and ratted out the other three in exchange for his own life. It's very possible that Marvin and Jules (and probably Mr. Wallace as well) had a few private meetings at some point, probably planning that morning's mission.

If you notice, Jules and Vincent stand in front of the door of the gang, waiting it for it to open at a specific time: "is it time yet?". Marvin is the one who opens the door.

My take on Marvin, is that he was a "new hire" of Marcellus and this was his first job for him. Maybe he was crapping his pants because he didn't realize how serious things would get, and of course he would be intimidated by his "superiors" Jules and Vincent, who have probably been working for Marcellus for years.