Galaxy Quest, 1999
The stars of a 1970s sci-fi show - now scraping a living through re-runs and sci-fi conventions - are beamed aboard an alien spacecraft. Believing the cast's heroic on-screen dramas are historical documents of real-life adventures, the band of aliens turn to the ailing celebrities for help in their quest to overcome the oppressive regime in their solar system.
First aired in 1999, written by David Howard, Robert Gordon, and directed by Dean Parisot
See Alan Rickman in slow motion: https://www.youtube.com/watch?v=eob7V_WtAVg