It may be possible to redownload the movie, depending on your country, but I would suggest you not count on it. If nothing else, movie studios have been known to remove movies from the iTunes Store with no warning. It's wise, and strongly recommended, that you keep backup copies of all your purchased apps and media.
For the procedure on redownloading, though, see:
http://support.apple.com/kb/ht2519
Regards.