I have an external monitor that I attach to my MBP to watch movies. Is there a way to turn on my screen saver or power down the screen on my MBP when I'm watching a movie on the external display?
What app are you using to play the videos? Many have an option for blanking other screens when playing a video fullscreen (for example, QT Player 7 has a checkbox in the "Present Movie" dialog, and VLC has a similar one in its prefs).
http://support.apple.com/kb/HT3131 that requires a keyboard
or you can turn your brightness all the way down, it will turn off your display but not the tv