Fake Brit: Australian Heath Ledger puts in a pretty convincing performance. Texan Alan Tudyk veers between pretty convincing, slightly over the top, and What the Hell Is That Accent?? (Of course, given the era the film is set, none of these is any more "wrong" than the others.) It's generally pretty convincing in the later scenes, Tudyk that is often mistaken for being Fake American.
The "We Will Rock You" opening was filmed by the second unit as a joke, and was kept (along with all the other rock music) because the team wanted to show that back then people thought their music was just as good as we think ours is today.
When Will wins his first sword match, the crowd is silent until Roland spurs them on to cheer. As it happens, the extras who composed the audience were Czech and didn't realize they were supposed to cheer when Chaucer finished his speech.
According to DVD commentary: Jocelyn's "Oh that is lovely!" in response to a priest's gesture prompting her to kiss his ring was an adlib. "Your entrails will become your extrails" was an audition adlib that helped Alan Tudyk get the part and made it into the film.
The knight that gets knocked off his horse in the opening scene of the film? That's Heath Ledger's stunt double, and the entire scene is actually footage of an accident he had while filming. He was genuinely knocked unconscious.