One theory is that the case contains Marcellus' soul. Those who support this idea offer the following points as some of the proof:
1. Marcellus has a band-aid on the back of his neck, and the Bible says that is the place where the devil extracts one's soul.
2. It glows.
3. The code to open the case is "666." 'Nuff said.
4. The men Jules and Vincent kill to get the case back are actually servants of the devil. Something went awry in the deal between Marcellus and the Devil, and it's "divine intervention" when all those shots miss the two hit men.
In actual fact, Ving Rhames has a scar on the back of his neck and Tarantino thought it would distract the audience in the ' over the shoulder ' scene.
THEORY No 2
This theory offers the suggestion that the case contains the loot from Tarantino's first movie, Reservoir Dogs.
Some critics reviewing the film when it was released, drew comparisons to the 1955 Robert Aldrich film Kiss Me Deadly, wherein the protagonist searches for a case filled with radioactive material. Hey, it glows when you open it!
THEORY No 4
But there is one theory that seems to make a little more sense than any of the others, and its name is . . . MacGuffin.
What's a MacGuffin? In a 1939 lecture at Columbia University, Alfred Hitchcock spoke of the MacGuffin. Crediting it to his friend Angus MacPhail, Hitchcock said it originated in the following exchange:
Two Scotsmen are riding in a train. One asks the other what is contained in a package in the overhead luggage compartment.
"It's a MacGuffin."
"What's a MacGuffin?"
"A device for hunting tigers in Scotland."
"But there are no tigers in Scotland."
"Well, then, it's not a MacGuffin, is it?"
Simply put, a MacGuffin is a plot device. It can be anything -- secret spy papers, a mysterious briefcase, etc. but its only purpose is to set the story in motion. Once that's accomplished, the MacGuffin usually become relatively unimportant. In the case of Pulp Fiction, chronologically, pretty much everything that happens with Butch and Marcellus (except for when they first make arrangements for the boxing match) occurs after the case is returned.
In an April 1995 Playboy interview, Samuel L Jackson, who played Jules, offered his perspective:
John (Travolta) did ask Quentin exactly what was supposed to be inside and Quentin said, "Whatever you want it to be."
As Humphrey Bogart's character said in Maltese Falcon (another classic MacGuffin, by the way), "It's the stuff that dreams are made of."
DID YOU KNOW .....................
Vincent Vega (Travolta) 'Pulp Fiction' and Vic Vega (Mr. Blonde) from 'Reservoir Dogs' are in fact brothers. As you've probably noticed, it's not at all unusual for Quentin Tarantino to have various characters, events, places, names, and storys from his films be intertwined. This is exactly what's going on with the Vic and Vincent Vega situation.
In fact, at one point Quentin Tarantino had even planned on making a film about these two characters entitled "The Vega Brothers". Unfortunately though, the plans for the film fell through over the years and it's now very unlikely to ever be made. This is because the film would have to be a prequel to both "Pulp Fiction" and "Reservoir Dogs" (for obvious reasons) and the actors are now well past the age of playing someone younger than they were over 15 years ago.
Vincent and Mia's waiter at Jack Rabbit Slims, who was dressed as Buddy Holly, was played by Steve Buscemi... you know, "that funny lookin' guy".
Buscemi was originally set to play the role of Jimmie (that Tarantino later took for himself) but his schedule wouldn't allow it. Tarantino still wanted Buscemi to have a role in his film though, so he cast Buscemi as Vincent and Mia's waiter during the Jack Rabbit Slims scene.
The irony in this is that in Tarantino's previous film, "Reservoir Dogs", Buscemi played a character that went on a long rant as to how he refused to tip inept waiters and waitresses. Vincent's line "I don't think Buddy Holly is much of a waiter" pretty much assures us that in the world of Tarantino... Buscemi would be one tip-less sonuvabitch.