Do you know where your story starts?

I've recently agreed to a collaborative writing project. The screenwriter had a "treatment" (like an outline for us writer folk) completed and wanted me to write a book based off of the four pages. Okay, I said, shouldn't be too hard.

I started writing where he thought I should. I went along with it. But after writing seven pages of word chunks in a few days (bad, bad, bad!), rather than flowing words, I figured that I really wasn't were the story started. I felt like I was cramming in too much in that first chapter and losing all of the feeling that the main character is experiencing.

I began to write a revised first chapter. I sat down and wrote six pages. Guess I found my beginning.

Do you ever start a project, write a while, and then decide that what you wrote isn't really the beginning?