Bill Murray, Saoirse Ronan and Tim Robbins on the City of Ember movie.

From SciFi.com:

The city of Ember itself was built practically for this film. In this era of computer imagery and green screens, what was it like to be immersed in a real world like that?

Ronan: We didn't actually have a lot of green-screen scenes. It was an amazing set that [production designer] Martin Laing had designed. And it was in Belfast. It was in the Titanic quarters: It was actually where the Titanic was built and painted. Or was it where it was painted? No, it wasn't. It was just where it was built. But, so, it was huge. And the whole city ... was actually like a little mini-city. So there wasn't that much green screen at all. ... We didn't really have to imagine what the city would look like.

Murray: Yeah, well, when you walk in and there's a street and an underground city that's 55 or 60 feet high with tunnels underneath it, and there's decaying doors and windows and bricks built as a street and concrete and plaster that's made its walls, real doors that open, real glass and beautiful design work, too, the emblems of the city built into the street and into the fountains in the street, fountains that work, ... it's not so hard to say, "I'm living in a crumbling society" when you walk in for work and there it is. It was so simple. And the costumes as well, ... the best costumes I've ever worn. The most beautiful I've ever seen in any film, by far.

(via SF Signal)