Between shots, the cast preferred to wait in the ship's lounge instead of their trailers/dressing rooms.
Nathan Fillion considers this to be the most fun he's had on a television series, and compares every other job he gets to it.
The Alliance's full title is the "Anglo-Sino Alliance." Joss Whedon intended for it to be the merger of USA and China, the last of the world's superpowers. That's why many characters sometimes speak Chinese. The Alliance flag, seen in the original pilot, is a blending of the US and Chinese flags.
The cast had a running gag where they would yell Summer Glau's name whenever they flubbed a line or messed up. It began after she forgot her line at the end of a particularly difficult scene. The gag continued through the filming of Serenity (2005).
Every time Mal rides a horse, no matter what planet, it's always the same horse. His name is Fred.