WALL-E is the character who's going to take you through the journey of the story and that world so you really need to relate to that character for him to take you over and get you hooked into seeing what's happening next. That's what Andrew really wanted to achieve - seeing how far he could go with one character and trying to discover a world through his eyes without reverting to the normal devices of delivering exposition or talking to another character. His friendship with the cockroach too, all of those are little details that makes that first part of the movie a special one.

To give you an idea of the way the film evolved during the process, the gel guys in Lifted - the short that opened with Ratatouille - I think there was a little cross-purpose there with WALL-E, because originally the humans in the film were actually aliens that spoke an alien language and there were all these other things happening. It wasn't until midway through that we realised that things needed to change and it needed to be us in the future. The notion of us being stranded on a cruise ship and everything else snapped into place and it felt that it had to be that way. In drawing terms, every six or seven months we would have a new version of the boards for evaluation. A tremendous amount of drawing went into making that revision. It actually began as more of a Spartacus movie, with WALL-E helping the robots stage an uprising against alien enslavement. It's changed quite a bit since then!

We arrived at the romance much later than the original concept. The sad, lonely robot was there from the conception. The character sat around for ten years and then, as I was writing on Nemo, especially during rewrites, I would beg for time at my office, throw pencils at the ceiling and procrastinate and I suddenly thought of this character.

They're designed to be opposites - she's a circle and he's a square. They are very much of the worlds that they're from. She's from this streamlined future and he's from this dirty past. Everything is encapsulated between those two worlds. They spent some time in each world and eventually they meet and their worlds meet at the same time, and that's broad enough that it could be read to be a symbol of their union or that those worlds collide.

I listen to a lot of soundtrack music and I remember I had Goldsmith's Star Trek: The Motion Picture score going. It just made sense to me - what's the opposite of loneliness? It's love. It's the point of living. Suddenly the only way to tell the story was to make it a love story.

For me the challenges with WALL-E go back to R2-D2 and problems I faced a long time ago - How do you get the human element into it without making it too human? How do you get the machine element into it without being too cold and impersonal? It's a sort-of 50/50 blend.