Wow. That must be, er, challenging...
On the other hand, the screenwriter has already done most of the heavy lifting when it comes to the plot and dialogue. My job is just to figure out how to translate the (unseen) movie into prose.
You also have to be able to check your ego at the door and remember that, ultimately, you're trying to do justice to somebody else's story . . . as opposed to trying to put your own stamp on it.