I just watched the movie again, came on TV, and was thinking just how and why did this thing suck so bad when the TV series was a huge success and Joss Whedon was behind both?
Because he wasn't behind both. The key is that movies and television are very different in their creative dynamics. In movies, the director and producers have creative control, and writers are treated merely as contractors, hired to do what the director and producers tell them to do. Writers have no control over what gets done with their scripts after they turn them in. Conversely, in television, the writers are the producers, and they have creative control.
So Whedon had no power over
Buffy as a movie. He wrote the script, and then it fell under the control of the director, the executive producers, and even the actors. Notably, Donald Sutherland insisted that his part be heavily rewritten to make him more prominent. So the film became a mishmash of what all those other people wanted rather than what Joss Whedon had imagined. It wasn't until
Buffy became a TV series, with Whedon as the showrunner, that he was able to do it the way he wanted.
By comparison, the reason
Serenity was so true to Whedon's vision and voice was because he was the director as well as the writer. That gave him a level of control analogous to what he had as showrunner on the
Firefly TV series.
