Likely cause: Your shader settings are too high for your PC.
Solution: Turn off shaders in the settings panel of the main menu. This will increase system performance at the expense of graphical quality. For best performance, turn off shaders to create a movie, and turn them settings back on to render out at full quality.
Likely cause: You have a lot of characters on-screen or a very complex set.
Solution: Reduce the complexity of your scene.