We all know that a movie is supposed to have one central protagonist. And the protagonist's actions drive the main plot of your script forward.
But what if you have a large cast of potential main characters? For instance, when you're adapting a TV series with a popular ensemble cast, or a novel or comic book series about a group or team of characters, who are by and large equally important? I'm thinking about The Avengers, The X-Men and the like.
Star Trek: The Next Generation, while having Captain Picard as its main lead, developed into an ensemble show over its seven years. Each member of the cast got to be the star in several episodes, so the viewers got to know everyone (almost) equally well.
When TNG made the transition to the big screen, however, it became The Picard And Data show. Picard had the main storyline, Data generally the most important subplot. The other characters were reduced to having 'character moments'. And naturally weren't all too happy about that.
So how to approach this situation from the writer's point of view? There are basically two major options:
1) You focus on a few members of the cast, giving them the most important storylines. The others become, for better or for worse, sidekicks. They will get their moments to shine, but their actions do not provide the main thrust of the narrative.
2) You focus on the goal the ensemble cast has to achieve. There are many important subgoals which must be achieved before the main goal can even be attempted. The members of the cast/team all have to meet separate challenges, which are given more or less equal weight. In the finale, the team comes together again to face the final challenge together.
It's obvious that the second option is far more difficult to pull off - and also far more plot-oriented than the first. The goals which need to be achieved need to be very clear, and the main antagonist/challenge should never be forgotten.
This narrative approach can be found in several TV series - the A-team always had something for each member to do separately. Also, in The Man From U.N.C.L.E., Napoleon Solo and Ilya Kuriyakin spent most of each episode apart before joining up in the final act and taking the villains out together.
And it also used to be a fairly common narrative strategy in superhero team books, especially when there was a cross-over with another superteam: the invincible villain captures all the heroes or charges them with an impossible task, the heroes are divided into small teams which each go off and have their separate adventure, and at the end everyone joins forces against the villain and defeats him and/or thwarts his diabolical scheme.
As will be obvious, these are all examples in which there is no character development. Rather, the nature of the characters is known and fixed, and the enjoyment comes from seeing them 'do their thing' and triumphing over impossible odds, in a spectacular fashion if possible.
The other option does allow for character arcs, and more character-driven storytelling. Of course, if you are doing an adaptation, you'll have to consider whether a character arc is desirable or necessary - this will all depend on the kind of property you're working with. Sometimes characters are at their best in their 'fixed' state, at other times the added potential for character development and evolution is a major plus.
In any case, if you have ten main characters and they all need to get an equal amount of screen time, it's obvious that there simply won't be room for any gradual evolution of any of them. They'll either have a very basic character development or they'll stay just the way they are and have always been.
No matter what approach you take though, plotting the movie will be even more difficult than usual. Because your protagonists are so plentiful, there will be less room and time for secondary characters. Each one of your protagonists will need to be essential to the plot of the film in some way, and will need to get screen time. The dynamics between the members of your team will be one of the main attractions for the audience, so you need to factor that in as well.
Therefore, a complicated plot is out of the question. There needs to be a clear conflict, a main goal which all the characters have to contribute to, and sub-goals which are tailored to test the separate characters and allow them their moment to shine - even in those cases where your main focus is on only a few members of the group.