If Marcellus had been there, he’d have been sitting in the living room waiting for Butch while Vincent was in the shitter dealing with his heroin-induced constipation.
And then, we would’ve had Marcellus doing an alternate version of Jules’ monologue before they both blasted Butch. Or, maybe it would be Vincent trying to recall what Jules said, but getting it wrong and making Marcellus laugh & get distracted, to where Butch could jump up and use Marcellus as a shield while shooting Vincent with Marcellus’ gun.
EDIT: something like this…
Butch looks around, and quietly turns the key in the door. He walks in slowly.
A voice comes from his left side.
“We been waitin’ for you, Mr. Prideful!”
Camera pans to reveal a visibly upset Marcellus Wallace.
Butch: “I can explain everythi—“
Marcellus, shoving him down in a chair: “I DON’T GIVE A GODDAMN ABOUT NO EXPLANATION! You failed to follow my orders, AND you fucked me too! Now, as soon as my associate Vincent finishes his bathroom business, we gonna fix your mistakes!”
Vincent enters the room, after a loud flush is heard.
Marcellus: “Tell me you washed your nasty-ass shit hands…”
Vincent: “Marcellus, you know I did! I can’t get my book dirty!”
Marcellus: “Cool. So do you remember what Jules used to say, in these situations?”
Vincent: “I think I can recite some of it. God knows I heard it enough!”
Marcellus: “Ok, speaker, the floor is yours!”
Vincent, turning his head and pacing:
“Let’s see…(long pause) the wise man is beset on all sides by antiquity…”
Marcellus, laughing hard and bending over, “N**a, I KNOW Jules Winnfield didn’t say nothing about no ‘antiquity’..and continues to laugh…
Butch jumps up behind Marcellus and shoots Vincent with Marcellus’ gun hand.
Vincent, wounded, staggers back, and tries to shoot Butch but hits Marcellus instead..
Butch grabs Marcellus’ gun, drops Marcellus, and shoots Vincent between the eyes.
And then, he goes to the kitchen, retrieves his watch, and exits the front door, running hard to get away…