I had to take screen shots of a dvd, and found this trick:

1. Go to the part of the movie you’d like to capture. Hit the Stop button in the DVD controller.
2. Press the Play button in the controller, and then immediately press down Command-Shift-4.
3. Hit the space bar to take a screenshot of the DVD Player window, then click the mouse to take the shot.

You can do all this without ever quitting DVD Player first.