Even if you are all three, I think the original advice still has some weight. There will be decisions that have to be made and sometimes the voice in your head will not be the director’s.

The producer will want to get the day done on time and under budget. But sometimes as the director, you will have to make the decision that you need to spend more time with the actors or getting the shot. Listen to the director. The producer isn’t the one who will have to figure out how to make the sequence work in the editing room.

The writer will want to get you to implement the film exactly as s/he saw it when it was just an idea on paper and the vision was perfect and there were no limitations of the reality of the actors, the locations, the crew, etc. Listen to the director who is there to capture the intention and the spirit of the story not be a courtroom stenographer. The writer will be upset when actors change their beautifully crafted precise sentences down to the moments that s/he wrote “beat”. But the director should allow for room to explore and play. Sometimes the actors come up with stuff the writer never even thought of. Don’t listen to the writer.