Every story has a hero, but no hero can survive without a mentor and an ally. The mentor helps the hero grow into a better person while the ally helps the hero survive in a new world. Then the hero turns around and helps the ally.
If you want to create a weak story, have every character help the hero for no apparent reason. That makes every character into a puppet who pops up long enough to help the hero and then disappears when they’re no longer needed. To see how this strategy creates a weak story, watch “2012” where the hero goes off to find his son after an ice age suddenly hits the world. For no apparent reason, the hero’s co-workers come along to help him and then die in the process.
For a much stronger story, make sure there’s a two-way relationship between the hero and the other major characters. In “Legally Blonde,” the hero is depressed and overwhelmed at being in law school, especially when she finds out her boyfriend is engaged to someone else. For emotional support, the hero goes into a salon and meets a hairdresser. The hero needs the hairdresser to help her feel normal again, but then the hero notices that the hairdresser has eyes for a cute UPS deliveryman. So now the hairdresser also needs the hero to help her get the UPS deliveryman.
Notice this two-way relationship? The hairdresser isn’t just helping the hero out of the kindness of her heart, but because the hero is also helping the hairdresser get her dog back from her ex-boyfriend and also help her get the UPS deliveryman. The hero still needs the hairdresser’s help in feeling like she has a friend so the relationship works both ways.
In “Star Wars,” Luke needs Hans because he has a ship to get him off his planet. Hans needs Luke because Hans needs money. Eventually the relationship between Luke and Hans will change, but they need each other so they have to work together.
If the hero doesn’t need the help of another character, there’s no point in having that other character. If that other character doesn’t get something form the hero in return, there’s no reason for anyone to help the hero. There must be a reason for both parties to benefit.
The simplest way for other characters to help the hero is because they’re friends. In “Get Out,” the hero has a friend who works as a TSA security officer at the airport. The hero relies on his friend to look up the mysterious disappearance and behavior of a black man that the hero has met at a party, but the TSA officer wants to help the hero because they’re already friends.
In “Die Hard,” the hero’s wife helps him because they already have a history of knowing each other and caring for each other. Of course, in most stories, the hero may not already have friends willing to help, so the hero needs to give these other characters a reason to help. In “Star Wars,” Hans helps for the money initially and then helps mostly to save himself and Luke from getting killed by the stormtroopers. Later, Princess Leia helps Luke because Luke rescued her.
Weak stories have characters who help the hero for no reason. Cut those characters out and only keep characters who have a reason to help the hero. That reason may later change, but initially there’s no reason for anyone to help the hero unless they get something out of it in return.