Most will agree that it is a sign of respect to accompany any agreement of creative or business collaboration with something in writing and a signatures. Even with friends or family who might initially brush off the idea as unnecessary. It is at the very least good practice. Even if a lawyer has not looked at it (and you can find some legal terms and wordings to reinforce an agreement), simple statements of intentions may at least give you peace of mind in the event of Rashomon or selective memory or simple change of heart. I would not mind initialing every page of a script as part of a contract obligating me to stick to what was written, but I often say it would be absurd to initial the ether to obligate myself to improvisation.
As a writer, I naturally want my writing vindicated. That can’t happen if I let the clashing objectives of others change or trade out what was written. Someone may like what are perceived as the broad strokes or story, but that is a container for the stuff I really care about, specific lines or images that may even subvert the expected message of the plot. There might be a format obligation to character arc, which means that either the start of a story or the resolution of a story presents a character as something other that he or she is best defined. Is Darth Vader a wise force ghost, fully redeemed, or is he best remembered as a petulant young man with control issues easily misled into becoming a functionary and then an iconic monster?
By the time I am asking anyone to be involved in a project, I have refined my script and generated storyboard sketches of camera decisions based on the beats of each scene as currently written. Others thrive on chaos. I don’t. It is heartbreaking to realize that someone in a room has intentions to throw a project off track and force you to approach the work their way. That’s when time has to be taken to have the hard conversation or to distribute a director’s note or manifesto or mission statement or anything that plainly states where you are coming from.
I would rather have no movie than have the wrong movie. I’ve gone through “the process” enough times to know that. Read the script, as much of a slog as it may be, and make an informed choice about whether you want to be involved with this movie. The only people cast in speaking roles will be those willing and able to learn the lines, rehearse, hit their marks, and get the pacing right. Most of the reading will be fast and flat, like a pebble skimming over the water, unless otherwise directed. Even if someone is an accomplished improviser, there should be no expectation of improvisation or co-writing. The biggest mistake a lot of movies, especially low budget, make is to have the script in flux.
The script will weed people out. It will choose who is the right fit for it, not the other way around. Anything I write is expected to alienate those on either the extreme right or extreme left of the political spectrum, and I plan to follow every problematic word of the script. That should be the default expectation going in. I will also follow the directorial plan I have story boarded. If that cramps anyone’s style, I’m sorry to have them withdraw from the project but I would be more sorry to have a creative tug of war or spend time on set fixing something that isn’t broken or placating an actor’s ego. The writer-director’s ego can be a factor but as the person who must take full responsibility for the final product (or lack of one) who does not wish to pass the buck over whose will overruled the original vision, he or she (or a grammatically incorrect they) has a concrete practical justification to stay the course.
You may like someone and think that you want to collaborate with or cast them, but ideology can be such a divide (even between center left and far left) that there is a danger of being stung along without respect for the vision at hand. It is far better to find out before dates are set that you have creative differences. I keep coming back to a Hollywood Reporter roundtable where Patty Jenkins described the first time Warner spoke to her about a Wonder Woman project and it was not the approach she wanted, and then Thor 2 which also fell apart for the same reason, but then even though more years had passed since her last feature the offer circled back to her once Warner Brothers came to be more receptive to her stated goals for Wonder Woman. She stated that you should drill down when you identify even a small difference between the movie you want and what a potential collaborator wants, because as the project progresses that small difference will grow and you will be making someone else’ movie.