I feel like, as a filmmaker, I'm at my strongest when I write the script and when it comes from me, out of whole cloth. My best work has always been self-generated.
For me, directing is like writing with meat. I can write live, in real time, and change things and be confident that I'm helping the movie.
In the writing phase, normally I try not to envisage any particular actors because I like to let the characters sort of reveal themselves in that process.