The crew only had about 15 minutes left to get the scene where the Doctor gives his big speech to the Devil in the pit done. David Tennant nailed it in one take.
The Beast was Russell T. Davies' response to producer Phil Collinson's observation that none of the Doctor's enemies during the first season had been godlike in scale.
Due to the shooting schedule (especially the fact that all of the episodes with the Cybermen needed to be shot together), this episode was shot closest to its air date. Combined with the immense amount of CGI work needed in this episode, the production staff was concerned about finishing it by the air date and, in fact, were unable to provide a preview tape to the media as they usually did (prompting speculation that they did not release a preview because of some big plot-based secret).
Torchwood Reference: When Captain Zachary Cross Flane addresses the Beast, he states that he is "representing the Torchwood archive."
Coincidentally, this episode was shown in the week of 6/6/06. It followed a wave of Devil-related stories in the media, including the release of The Omen (2006).