The question that you get asked most frequently as a writer is "where do you get your ideas from?"
Douglas Adams used to answer, "From Ideas-R-Us near Swindon."
I think Stephen King says, "If I knew where they came from, I'd go there more often."
One thing I've noticed is that ideas are sometimes attracted to other ideas.
When Carl and I first started talking about doing a very low budget film, I remembered an idea I'd had ages ago about a boy who had gone missing and the effect it had on his family.
I blocked it out - it came out as three short stories and it seemed very strong.
I used a lot of repetition in the stories because I thought a/ it would make things cheaper (re-using locations) and b/ it would be quite eye-catching. We haven't got money for lots of great locations or special effects, but unusually formal storytelling can be just as exhilarating as these things - eg. Cache.
I used a recurring Scrabble game as one of the motifs. This led to a lot of conversations about Scrabble, which in turn led to Carl going off to a proper Scrabble tournament in Durham. He's come back with terrific notes and interviews. The material he's found is so strong that I'm now thinking of junking the original story (but not the characters) and writing a newer, simpler story that will give us space to enjoy the strange world of Scrabble.
So one idea led to another idea that swallowed up the first. A sprat to catch a mackerel. Hopefully.