What significance did 9/11 have in the 25th Hour?

The movie is based on a book written prior to 9/11. But since the movie was shot afterwards, Lee decided to include it "in a millisecond", because

• Lee is a New Yorker
• It was a traumatizing event that changed the world and they wanted to reflect that

Edward Norton said that it was "now part of the fabric of New York" and else it would have looked like "an insane kind of denial."

From a 2014 interview with Stella Papamichael for the BBC:

Question: "25th Hour" is the first film to capture the mood of America post 9/11. It's not a feature of the book (written prior to that), so what made you decide to include it here?

Lee: We never saw it as a race to be the first 9/11 film. It didn't matter if this was the first or the hundredth film to do it; it didn't change our feeling about this. What happened on September 11th really traumatised Americans and we're still feeling the effects. New Yorkers, and Americans generally, are living in a much different world now. The threat of terrorism is an everyday occurrence, so we wanted to reflect this different world. It's that simple.

From a 2002 interview with Wilson Morales for blackfilm.com:

WM: This film was based on a book that was written before 9/ 11 and it could have been easy to not shoot the horrors of the aftermath, but you chose differently. Why so?

Lee: Cause I am a New Yorker and a couple of studios had a chance to show stills of the WTC but they chose to punk out. The project was based on the bottom line. I don’t think they should fear the sensitivity of the movie going audience. I don’t think “Spiderman” would have made a nickel less if they would have kept those images in, but that’s their decision and on this film I was able to implement my decision and I would like to add that the decision regarding 9/11 was not a big decision. I made that in a millisecond. I knew I was going to do; I just had to think how I was going to do. That was a much bigger and harder decision because I didn’t want to offend anyone and we still knew there was a way to deal with it in a tasteful way but not run away from what happened.

WM: Did you think it was relevant to the story?

Lee: Yes, we did not want to do something that looked like it was slapped on.

Edward Norton in a 2014 interview with Stella Papamichael for the BBC:

Question: What about the decision to set Monty's story against the aftermath of 9/11?

Norton: Of course there's a political context to that event, but there's also an emotional context that has nothing to do with politics. It's part of the fabric of New York now. If I felt at any time that we were losing the focus of the story and digressing into a political comment, I'd have been very uncomfortable with that. Not that you can't make a political comment on those events, but that's not what this story is about.

That same summer we were shooting in New York, they were making "Maid in Manhattan". OK, you're not going to point the cameras at Ground Zero if you're making "Maid in Manhattan", but we were making a film about loss and about the consequences of choices, and taking things for granted. To not allow that new emotional reality into the background just seemed like an insane kind of denial.