The best movies tend to have a growth arc for the main character. In the end, they have often somehow become better versions of themselves, as well as having solved some big problem in their world. This means they have to start the movie as the “not best version of themselves”. And this is where many scripts I read run into a problem.
If the “not best version” of your main character is a selfish jerk who hurts others in some way, readers will tend to not bond with them enough to want to stay with the script. And I believe the single most important factor for audience engagement is forming that connection with the main character. I also think that’s the primary function of the first ten pages — to start that process, then really cement it when the “Catalyst” happens, in mid-Act One.
In search of a suitable character “flaw”, however, many writers start their main character pretty unlikable in terms of their treatment of (and caring about) other people. Then by the end of the movie, the idea is that they will care about others and treat them better — so that they have a satisfying character arc.
I believe this almost always undermines readers’ ability to care enough about the main character to want to stay with the story. There’s a reason why the title of Blake Snyder’s famous book focuses on the main character needing to do something selfless to make the audience care about them, in the opening pages — such as “save a cat”. Because it’s how characters treat others (usually people) that most determine how much the audience bonds with them.
It’s really helpful in creating that connection for the audience if the main character not only does something nice for someone else, but does it as a sacrifice of their own wants in the moment. It’s not enough that they’re just generous in some way that doesn’t cost them much. They have to actually put others in front of themselves in some way. If you really look at the most successful movies, I think you’ll find that they really go out of the way to make the main characters people we can respect, like and want to follow — often through that technique.
This tends to not be as necessary (but is often still helpful) in movies where the main character faces huge life-and-death stakes right away, and/or is heroic. We can forgive a lot in those cases. But if their problems are something less than that, and/or they are not heroic, we tend to not want to follow them if they are basically a “jerk”. We’re much more interested in following relatable underdogs.
It’s true that there are a rare handful of movies where the whole point is that a “jerk” becomes a better person. A great example of this is Liar Liar. The main character is purposefully presented as someone who is not particularly good to other people, because the whole point of the film is that a magic spell will force him to tell the truth and be “better”. That’s the one time where you can get away with the “jerk” opening.
But even there, you’ll notice that he’s actually really good to his kid, when he shows up. His problem is just that he’s unreliable and evades the truth. But when present, he’s a great, loving dad — right from the opening pages. So even in this movie about a “jerk who stops being a jerk”, the filmmakers go out of their way to give him some positive qualities from the outset, in how he interacts with and loves his son.
Also, this character is REALLY entertaining to watch, which helps — the movie is hilarious from page one. And, importantly, he gets completely beaten up by life, for most of the movie. They aren’t life-and-death stakes, but he’s being punished for almost the entire film. We can forgive bad behavior more when the main character is punished for it in a big way, pretty much constantly.
This is often done with womanizer characters, like Vince Vaughn in Wedding Crashers. He’s unrepentant in how he uses women, but he gets taken to task for it, almost from start to finish. His happy success at this is only evident in the opening montage. The rest of the time, he’s paying a big price for it. He’s also not the main character. Unrepentant womanizers usually aren’t. The main character, played by Owen Wilson, has more of a conscience, and is focused on a particular woman who we’re led to feel could be a true love connection. (Where his interactions with her have heart — like when he helps her with her toast at the wedding.)
My point here is that the main character’s “flaw” should not be that they hurt others in some way, out of selfishness, almost ever. Instead, I like to think of the flaw as the way the main character “gets in their own way” — usually through limited thinking about what’s possible for them. This flaw might have a side effect of giving people around them a less-good life than if this character was fully self-actualized, but they are not directly hurting others in a way that we see on screen and blame them for. Instead, they are living a compromised version of what their life could be, because they somehow haven’t risen up to face the internal blocks that are keeping them stuck — which is what the big challenge of the movie will force them to do. But they’re not jerks!
If you haven't read them already, I also recommend my "Ten Key Principles Successful Writers Understand", and my series of audio downloads.