Ben Barnes (Young Dunstan Thorn) was a fan favorite and the preferred choice for Tristan over Charlie Cox, as he was considered more attractive. Matthew Vaughn intentionally cast the then-unknown Cox over big name actors like Orlando Bloom, as he wanted an actor who could play a dork and easily transition into a suave and handsome gentleman.
(at around 1h 16 mins) Much of Ferdy's dialogue was ad-libbed by Ricky Gervais (for example, telling Lamia he can get her a two-faced dog as a guard dog that can watch front and back entrances at the same time).
Miramax originally had the movie rights. After they expired, author Neil Gaiman felt uncomfortable granting the rights to just anyone. After turning down numerous directors and young actresses who wanted it as a starring vehicle, Gaiman finally granted Matthew Vaughn the rights for free. Gaiman trusted Vaughn, both as a friend, and as someone "who stuck to his word", something Gaiman considers a rarity in Hollywood.
All Princes wear clothing with a pattern spelling out their number in Roman numerals, composed by smaller Arabic numerals. Furthermore, Septimus wears a vest with the numeral seven on each button.
Matthew Vaughn had great difficulty shooting the scenes at the inn where Lamia entices Yvaine, Tristan, and Primus. There were very few days when more than one cast member was available for filming. He used a lot of stand-ins in the closer shots, to give the impression that all of the cast members were present.