Check out Bryn Donovan‘s list of idea starters! She made it with novelists and short story writers in mind, but they’re helpful for script writing, creative writing teachers, and even for improv coaches.
No matter what you’re working on, the way you use these writing prompts is basically the same: you pick a line of dialogue and you write a conversation or a scene that includes it. Most of these are geared toward contemporary mainstream stories, but I’ve thrown in a few that go more in the direction of historical, mystery, thriller, romance, fantasy, and science fiction.
It took a long time to come up with these, but it was also pretty fun. If one of them winds up in your finished work, that’s just fine with me. Writing prompts like these can be really helpful if you sometimes wonder how to overcome writer’s block.