What is the name of that movie in which a writer whatever he writes comes true?

You're likely thinking of the movie "The Secret" (2006). While not a movie per se, but rather a documentary film, it explores the power of positive thinking and the idea that what you focus on, you attract into your life.

However, if you're thinking of a movie where a writer's words literally come to life, then you might be thinking of one of these:

* "The Last Wish" (1997) - This dark fantasy movie features a writer whose words can create reality.

* "The Number 23" (2007) - While not directly about writing, the protagonist becomes obsessed with a manuscript that seems to predict his life.

* "The Secret Window" (2004) - This psychological thriller features a writer who believes he's being stalked by a mysterious figure.

If you can provide any more details about the movie you're thinking of, I can help you pinpoint the exact one.