Most of the game's cutscenes are real-time in-game animations, but the inXile splash movie, the game's story introduction and the end game movies are separate video files.
We've discovered a problem where sometimes QuickTime chooses the wrong Codec for the .mp4 movie format we used. We have changed them all to .mov in a previous update.
If this issue still occurs, you can resolve the issue by updating QuickTime to the latest version and removing external codec packs and video players you may haev installed (such as Perian).