When you start to use compiz-fusion and all the fancy desktop effects you will notice a little problem, you can't watch videos!. There's a bug that crashes mplayer, totem and all your video players. Hopefully there's a workaround for mplayer (not sure how to solve the problem with totem).
1. Open mplayer.
2. Right click over the window.
3. Click on preferences.
4. Click on the Video tab .
5. Change the video driver from Xv to X11 and click Ok.
You may have to restart mplayer to see the changes.