Movies always make the hard part of writing seem like coming up with the idea. There's always some author who is struggling with writer's block and doing all these crazy things to cure it while her agent nags her about whether or not she's done yet, reminding her of the deadline that's already passed.
In reality, the hard part is actually having the motivation to sit down every day and write. It's trying to juggle life and at the same time develop a following on your blog and on twitter while you polish a novel that you've been working on full time for the past two years. It's being patient when you see little results and okay with the fact that no one is going to praise your work or even read it probably until a long way down the road. It's struggling with loneliness and getting people to care enough about your book to buy it when you're finished. People aren't holding their breath, waiting for you to come up with the next greatest idea and even if you do come up with something, it doesn't mean much if you don't take the time to polish it and present it in as flawless of a manner as possible.
If you have an idea for a good story, don't let it just sit in your head. That's not what writing is. Take at least an hour a few times a week to work on writing your idea. Don't worry about it coming out perfectly. My novels always read like a second grader just wrote them in my first draft. You write and rewrite until they transform into what you imagined.