Every screenwriter has his limits

No matter how good you get as a writer, you can't expect to write everything.  There are certain styles, subjects and tones that a particular writer might be excellent at, while being completely inept in others.  In fact, the best screenwriters are probably those who understand not just what they're capable of, but also what they're NOT capable of.

Take Aaron Sorkin, for example.  In this Daily Beast article, he talks about a conversation he once had with Steve Jobs:

[Jobs] wanted me to write a Pixar movie. I told him I loved Pixar movies, I’d seen all of them at least twice and felt they were small miracles, but that I didn’t think I’d be good at it. 

STEVE: Why not? 

ME: I just—I don’t think I can make inanimate objects talk. 

STEVE: Once you make them talk they won’t be inanimate. 

ME: The truth is I don’t know how to tell those stories. I have a young kid who loves Pixar movies and she’ll turn cartwheels if I tell her I’m writing one and I don’t want to disappoint her by writing the only bad movie in the history of Pixar.

 Most writers would kill for Aaron Sorkin's talent... and yet, there are still things he's afraid of.