If nothing else, you copy the phone book, or the Bible, adding a word or two of your own now and then. Another approach might be to tear a page out of a book or newspaper, cut the page into pieces, rearrange the pieces and see if any interesting juxtapositions occur that might beg for elaboration. Sound editing and design can use the same sort of trick. Eventually I hear a sound that makes an interesting connection between two moments, characters, or places in the film. That is what sound design is about.
Most people think sound design is making alien vocals, ray guns, and space ship sounds.
The same people probably think that film editing is about cutting stuff out of the movie that's boring. Mistakes, accidents, and the unexpected often provide the spark that leads to great work. The trick is to plan and execute your creative process in a way that makes room for lots of experimenting and lots of mistakes early. Actually the best reason to rehearse is to discover what needs to be done. Creating implies making something out of nothing.