Director is the main influence. It is directors who make the great actors and in most cases discovers them. Writers are important, but not in all movies; some movies are just a series of chases and fights; the director and the stunt choreographers would be the main influences there. Some great actors become great directors, which is why they make that move. Some actors can't do anything but look good on camera for maybe 30 seconds to a couple of minutes, and read lines off cue cards. Editing makes many of them look good to audiences, and it is directors who do most of the editing. In early Hollywood directors would have be effective thugs to get the crews in line.
I agree. If I picked one person most responsible for the success of a movie, it would be th director and here's why
When a director is hired, he is given a script, a budget, a list of potential actors for leading roles and any non-budget requirements by investors. The Director and Producer meet with possible candidates for leading roles. The Director picks who he wants for leading rolls and the Producer works out contractional details. Later a casting manager will be hired to fill the remainder of the cast.
Then comes the planning which is usually the longest phase of production. Where each scene in the script is broken down into many scenes as seen by the camera. For example this one scene in the script is actually 9 scene when filming.
Mike and Diane are in a conversation. CUT. John enters the room laughing sitting down in a chair. CUT. The conversation continues. CUT. Camera shows the window where two bullets are fired. CUT. Camera goes to John laying on the floor covered in blood. CUT. Camera goes Diana screaming in horror. CUT. Mike jumps up running to window. CUT Camera goes to the door where the Butler enters the room. Cut. Conversation between Butler and Mike who tells the Butler to call the police. Cut.
Typically, there are serval dozen to a hundred scenes in a movie and all of them must be mapped out. That mapping includes how each actor plays each roll in each scene, settings camera, lighting and sound positioning, and designs of sets and costumes.
After shooting, the Director will work with the film editor to created the Director's Cut. The final cuts will be made depending on the customer requirements.
Also the Director will alter script in each scene as needed. What is own paper often does not work on film. Lastly the director must make sure that the acting in each scene meets his vision for the film, not that of the actors as actors will always try to enhance their performance.
If the movie is a great success, it is the Director that will receive the awards for the movie and if it is a failure it will be his failure.