Hobbit, Peter Jackson, behind the scenes
What Peter Jackson describes as the "best scene" in "The Two Towers" wasn't directed by him.
Jackson: Fran directed probably the most famous scene in ‘The Two Towers,’ which is the scene where Smeagol and Gollum talk to each other at the camp when the two hobbits are sleeping. It was the scene that sort of established the whole schizophrenic idea of the character, and it was a very late idea that Fran had. We shot most of the movie, but Fran said, “Well, we haven’t nailed the Smeagol-Gollum relationship really.’ It wasn’t really that clear, and she wrote this scene that made it very clear, except it was just the very end of the shooting, and I didn’t have time to shoot it because all my days were full with things that I had to shoot that were already scripted. So we built a little set and we got a tiny crew together, a very small crew, and we said to Fran, ‘Well, if you want this scene in the movie, you’re going to have to shoot it yourself.’ And Fran shot that scene, and it’s actually probably the best scene in the movie.

Philippa Boyens says it was Jackson's idea to turn the two films into a trilogy.
Boyens: I think it was Peter. I think he suspected, but it did come to all of us together when we watched Pete’s first cut of the first film. I think we had a sense of the fullness of what we wanted to tell and what was left to tell and what you could tell and the possibilities of what the story could be, but we were actually surprisingly being quite conservative and I know Fran and I especially wanted to know that it was playing as a story first. You have to trust that, you have to feel are people are going to want to invest in this story? Does it have enough juice? Do you care about it? The whole sense of Azog the Defiler, I mean, just the name was enough where you want to go there and you want to tell that quite powerful story about the hatred between dwarves and orcs and where that comes from and being able to do that for example. And also the great thing about working with Pete is that he never gives up on a great idea. He never lets go of it. Even if you turn up the night before and say, "I know we wrote the scene, and I know it’s this way, and I know you’ve blocked it, but we had this thought." The great joy about working with him is that he will do it. He will find a way to make it happen. And I think I know from a lot of my friends who are writers, that’s very rare in a director because being able to be that imaginative and that brave is rare.

The Hobbit: An Unexpected Journey, Richard Armitage,
The filmmakers weren't worried about losing an additional year of their lives by expanding the the scope of the films from two to three.
Jackson: It was actually almost the reverse. Because as Philippa was saying that we had other ideas for the story that we could put into three movies if we went there. We are going to have to shoot for another 10 or 12 weeks next year. We haven’t shot three films, but we have these ideas about how it could become three from two. Really the thought was that we’re not going to come back to this world again. This is probably our last journey to Middle Earth as filmmakers, so it’s now or never. We’re not going to be able to in two or three years time, go to the studio, "Hey, could we do these other ideas?" Because it just wouldn’t make any sense as a standalone film. What makes sense is we could work them into the three as a continuous story. So we literally had to figure out: do we put our hand up now and do it, or do we regret it?

Jackson says the studio's response to an extra film was a "responsible" one not just looked at for commerce.
Jackson: It was certainly interesting to see the studio’s face when we suggested three movies. They had no idea it was coming. They were very responsible actually. They said, “Look, we like the idea, we’ll support it, but we gotta know that it’s going to work as three individual movies, that it’s not going to be just padded out or stretched out." We’d had mapped it all out on paper. The three of us had worked out a restructure on paper of how we’d actually get the beginnings and ends of these films and how the shape and the emotional arcs would work within the three films.