He was so frightened by the film that he couldn't get to sleep.
He was so frightened of the film that he couldn't get to sleep.
1 Answer
Being frightened by a film means to have been frightened by the content of the film. A single image or scene could be what was considered frightening, or perhaps the film as a whole due to ideas or subject matter presented.
I believe it is much more common to be frightened by a film, and if the person the sentence is directed to is familiar with the title or genre of the film, it's likely that no further explanation will be necessary (for example, it's common for people to have nightmares after a horror movie).
To be frightened of a film is less common, and will likely require an explanation. I can think of two examples:
Characters in the movie The Ring died seven days after viewing a particular film. While the subject matter of the film they viewed was strange, there was nothing particularly frightening in it. Their fear was of the film itself, due to their belief that it would kill them a week later.
When I was younger, my mother refused to let me see The Exorcist - she wasn't particularly religious, but very superstitious and believed that someone who simply watched the film might become possessed. I don't believe she had seen the film herself, and so, "My mother was frightened of the film The Exorcist because she believed it had the power to cause possession.
I hope this helps!