Newsweek columnist N'Gai Croal interviewed Bungie's Joe Staten about the Halo 3, Halo 2, and the Halo movie back in August of 2007. The first part is online now. Croal picked out one blockbuster revelation by Staten to highlight in a second post prior to the publication of part two, however: that the faceless Master Chief wasn't really going to be the film's protagonist.
In the film, the other characters begin to comment on Master Chief's anonymity, like "Who is he?" and "What's his story?" He becomes a really wonderful source of mystery, a sort of anonymous problem solver. So we definitely worked on that. In the final version of the script the Master Chief was certainly absolutely critical to the film, but there were other characters around him which carried most of it, that did most of the emotional heavy lifting. The Master Chief was there in support of their story.
Staten also does confirm something that many fans have long suspected, and that even some people "in the know" have expressed doubt at: that Halo 2 was supposed to be the end of the story. Staten says, "we didn't think we were necessarily going to make Halo 3. I mean, we made Halo 1 not knowing we were going to make Halo 2. So we started out designing Halo 2 not thinking that we were going to make a Halo 3." Halo 2's third act, even though it was different from what we got in Halo 3, could have ended the story. Halo 2's cliffhanger simply made it impossible to stop there.
All those who think there must be a Halo 4 should take heed of that.