Dawson Casting: One of the two 12-year-old leads is played by Saoirse Ronan, who was 13 at the time of filming. The other is played by Harry Treadaway, who turned 23 during production.
Doing It for the Art: A set nearly as big as a small town was constructed to stand in for Ember. It was so big, most of the cast needed a map to get around.
Enforced Method Acting: For the scene where the Mayor is interrogating Lina, they filmed Bill Murray's coverage first and then Saoirse Ronan's. When doing the latter, Bill Murray would go off script and improvise new lines to keep Saoirse looking confused and surprised.
Both Lina and Doon. Lina is played by the Irish Saoirse Ronan, and Doon by the British Harry Treadaway.
In the supporting cast, there's also Brits Toby Jones (Snode), Simon Kunz (Capt Fleury), Mackenzie Crook (Looper), Liz Smith (Granny), Marianne Jean-Baptiste (Clary) and Lucinda Dryzek (Lizzie).
Follow the Leader: Among the many adaptations of children's fantasy/adventure literature in the 2000s, following the lead of Harry Potter and The Chronicles of Narnia. There are a couple of other similarities to City of Ember and the Narnia films; both produced by Walden Media, both the sophomore effort from a director who'd released a successful animated film (Shrek for Andrew Adamson, Monster House for Gil Keenan), and Harry Treadaway even looks a bit like Skandar Keynes (who plays Edmund in Narnia).
Stillborn Franchise: Part of the deal to get the rights included an option on adapting the second book The People of Sparks. The film's Box Office failure meant that no sequels were made. The film does at least end on a note that makes the story feel somewhat resolved.
What Could Have Been: Sets were built for the movie that ended up not being properly used. For example, a hairdressers clinic was constructed but only appears in the background.