What is the most critical element of film making?
Truthfully all elements are critical.
You have 3 phases or stages in the creation of a movie, TV show, etc... They are: Pre-Production, Production, and Post-Production.
At any given point there are hundreds of things happening in each stage of a movie's development.
As a cinematographer (Director of Photography), I will work with the director through all three stages. In pre-production I work with the director and producers to understand what will be needed for each shot and scene in terms of lights and cameras. I may need to work with the SFX coordinator to develop how special shots will be handled. All of this is planned in advance. You never get to the set and then decided what is going to be done.
It's all handled in pre-production.
My role can be greatly different from project to project depending on the director. In a heavy SFX film the director may be very involved in details such as lens choices and camera angles.
In other projects the director may give me great latitude to decide everything from what lights and where to camera direction/movement and angle. It just depends on the project and the director's primary focus.