In Chamber of Secrets, both of the Vanishing Cabinets (one in Borgin and Burke's, one in Hogwarts) are mentioned for the first time, with very little attention drawn to them. They seem to be completely unimportant objects, right? Yet, continuing through the books, they (at least the one in Hogwarts) continue to make apparences. By Half-Blood Prince, there can be no mistaking that they are, indeed, objects of importance. But look at the movies...
In the movie CoS, I seem to recall that HP does indeed hide in a cabinet at B&B, but I have no recollection of the Hogwarts cabinet (which ends up being the more significant of the two) ever being mentioned. I haven't seen the film version of OOtP yet--is there any mention made of Montague being trapped in the cabinet? I imagine it was cut, simply because it could be. It doesn't really distract from the continuity of the storyline, even given what we know about the cabinets' roles in HBP. There are going to be far more nuances and outright clues in the books than in the movies, or the movies would run to ten hours apiece.
In a way, they are the same story being told to two different audiences--but they are the same story.
Another reason why I have not yet dismissed the possibility that the locket mentioned briefly as being present at Grimmauld is indeed the Horcrux--RAB. There is no doubt in my mind that RAB is Regulus Black; I think Rowling has admitted it (retrokitten might know for certain). The note left in the fake Horcrux is obviously significant, and Regulus can certainly be tied to Grimmauld Place. He was the favored son, after all. Not only that, he left the note knowing that his death was imminent. I think it would be natural for him to return to his family, to the safety of his own home, if for no other reason than for the comfort it may have offered him. The Death Eaters, IIRC, had no idea at all that LV was creating Horcruxes; LV himself does not seem to have known that RAB took the real Horcrux. Regulus was being hunted down by the Death Eaters simply because he got scared and wanted out--no other reason is mentioned, or LV would have ransacked Grimmauld Place himself in search of the missing Horcrux.
My, my, I do go on! Was Montague trapped in the cabinet in the movie? Inquiring minds want to know!
Nope...
The general feeling is that it is Regulus Black, but she doesn't come right out and say it (unfortunately!). BUT wouldn't it just be like her to stick something in there to throw us off? ;-)
From the Leaky Cauldron/Mugglenet interview:
MA: R.A.B.
JKR: Ohhh, good.
[All laugh.]
JKR: No, I'm glad! Yes?
MA: Can we figure out who he is, from what we know so far?
[Note: JKR has adopted slightly evil look here]
JKR: Do you have a theory?
MA: We've come up with Regulus Black.
JKR: Have you now?
MA: Uh-oh.
[Laughter]
JKR: Well, I think that would be, um, a fine guess.
MA: And perhaps, being Sirius's brother, he had another mirror -
JKR: [drums fingers on soda can]
MA: Does he have the other mirror, or Sirius's mirror -
JKR: I have no comment at all on that mirror. That mirror is not on the table. [Laughter from all; Jo's is maniacal.]
MA: Let the record note that she has drummed her fingers on her Coke can in a very Mr. Burns-like way.
[Laughter]
JKR: Oh, I love Mr. Burns.
I was surprised that Umbridge asked for students to volunteer to be members of the Inquisitorial Squad.
Didn't see enough of them in action to make it worth shoving Montague into the cabinet.
There are going to be far more nuances and outright clues in the books than in the movies, or the movies would run to ten hours apiece.
The whole thing really should have been done as a TV series, at the rate of 2-3 years per season there would have been enough time to do it justice.
Nope!