When my friend and I watched certain movies, sometimes we'd have the critique "too many characters" or "needs more character development." One of these problems we could diagnose and solve, but the other was a little more complicated. How do you define character development? Is it quippy banter, or extra dialogue explaining their favorite color? Is it those scenes where the characters connect on a deep emotional level, or where the characters fight each other over something? It could be any of these things. But, saying a character or story needs those scenes or dialogue is missing the point. At the core of any of those examples, of any scene and any story are choices. Choices define characters and have the greatest impact on the progression of a character driven story. But, it's not just the big choices, like setting off on a quest, or saving the princess, or sacrificing yourself. In fact, the smallest choices, the choices that aren't so obvious can play just as important a role. These tiny choices are what seperate strong characters from weak ones, and great actors from not so great actors. But, above all else, the small choices are the answer to the problems of too many characters and needing more character development.
See, in a novel, you can take your sweet time delving into all of a character's likes, dislikes, hopes, dreams, goals, occupations, what their fantasy football team is before you even start the story. In film, this can't happen. The story has to start almost instantly. In a novel, you can pick it up, set it back down, and come back to it later. In film, you have sit in a dark room surrounded by strangers for about two hours on average. There's a big difference between the two. And, in film you don't have time to go into a lot of detail. However, film gives you the option to see what's going on. This is huge when used properly. You can observe a character's posture, the way they walk and navigate a space, how they handle objects, how they act around each other, how they act in different situations, facial expretions. All of these take up very little screen time and tell us an enormous amount of information about a character, more so than with dialogue alone.
As writers, looking for the tiny choices liberates us to explore the characters more and come to understand them better. As an audience member, we gather more information from what we see rather than what we hear. So, when we see a character's posture, the way they handle an object, or the tone of their voice used, we come to understand them better. And, you don't have to put every detail into your script. Just give the actors something else to play with or do, adding an extra layer of communication.
All of this is how you solve the question of multiple characters. It's easy to write all of these choices for one character. But, when you have several key characters, or a multiple protagonist movie, you cut down on the number of scenes some of the characters have to shine, unless you give them as much to shine about as possible in every scene, which you do with them making as many choices as you can give them, whenever they're in a scene. So, when you have two characters having a heart to heart moment (which tells us something about the dynamic of their relationship) give them something else to do that tells us more about them individually.