Hello julesclark,
It all depends on the format of the movie. Rather than installing additional plugins and extensions, give VLC player a try. It's free and in all the years i've used it i have not found a format it can't play.
If you do want to stick with QuickTime, have a look at Flip4Mac and Perian
If you have VLC and double-clicking the email attachment does not work, try a right-click > open with VLC, or save the video to your desktop first.