How can you create a consistent character arc for a movie?

A great way to "know" your character is making a connection you know personally or of someone you have studied, admired, or had an interest into what makes them unique, tick, or fuels their engine. EX: An old school writer would go cookie cutter to write a villain like Lex Luthor in a Superman movie and it worked then (look at the example of Lex in both Superman'78 and in Superman Returns Lex's Land scheme, not so successful!). For Batman V Superman had they made Lex more of a Steve Jobs character than Zuckerberg, he'd be more relatable, and not fall into the "villain knows all just go with it" bucket. A Hero always does what's right not because they can but because they should, a villain is NEVER the villain in their own mind.

Positive change arc type - NEO in The Matrix, Luke in Star Wars. They start as one thing and end up as something they could never predict themselves as at first. Negative change is Michael Keaton as Faye Krogg in The Founder, a simple man who wanted more, but became the villain. A Flat arc type is Marty McFly in BTTF and Indiana JOnes in Raiders. Both are the same in the beginning and end of the film, they haven't changed or learned essentially, but are the driving force of change for everyone else as they jump into their adventure , becoming the disruption to the story (in essence, Indy is the Rolling Ball to the Nazi's searching for the Grail).

A film to study for plotting Key moments would be the recent Mission:Impossible films, not many films succeed in this where every decision that has to happen NEEDS to matter, and the consequences are felt throughout. Another perfect example of this in writing is the game series MASS EFFECT (1, 2 &3) where your decisions snowballs into either bigger problems politically or story wise (Ex: choose to save the council or the citadel, save the Krogan or not, etc). Those choices not only make the game different for you later, but affects how people react to you later in the game because you made those choices!

Showing change can be visually large, either a character loses a limb, and eye, gets a scar, a typical massive visual que to their growth. Or, a sublte perspective change showing either acceptance or not, of a person/ideal/team, etc. Perfect example here is Gimli and Legolas in LOTR, didn't trust each other at first but accepted a compromise to protect the ring, and at the end of that journey, they became brothers in arms.

Do NOT let your MOM read your script. Do NOT let your Family read the script. Do NOT let your loved one read your script. DO let someone you know that you TRUST that will always tell you what you NEED to hear RATHER than what you WANT to hear, read the script, to give you an honest opinion and feedback. Now if they say "I don't know why but that part doesn't work" LISTEN TO THEM> If they then say "You know what you do, you make them do this....." DON'T listen to them! They're not the writer. YOU ARE. The fact that they're telling you there is something off that needs your attention to fix, go with that and you'll find the fix you need.