I've encountered this problem twice in the past couple of weeks, once in my own writing and in a fellow writer's work at group. You have your story all laid out. The next move your character makes is to charge into a burning building...but why?
(please keep in mind, these are made up examples)
My writer friend had her MC charge back into the building to get his phone. This method of getting him into a flaming house made him look...
Neither of these fit with her character, who is actually very intelligent and not materialistic. But she did want him to come off as a little naive so I suggested she have the antagonist trick her MC into the building by saying there is a little girl trapped in there. Her hero rushes off to save the girl, discovers he's been fooled, feels like an idiot, and we, the reader, feel terrible for him (because we've all made mistakes like that.) We also love him for being so selfless to try and save a little girl, and we hate the antagonist that much more for tricking our hero into the burning building.
In my own example, I had everything figured out. My MC was going to rush into the building to save the little girl on his own, because he was the typical all-American selfless hero. Then, tooling around on Goodreads I found a review that struck me.
Oh goody, another book about the all-American good guy, running into save everyone.
She was right. It was probably new a hundred years ago, but it's old now, and struggling to break into a competitive market, it's imperative to be different. I didn't have to re-think my entire plot, only the motivation and character aspects. Instead of my MC going into the burning building on his own, I had him dragged in there by my antagonist. Now I have something interesting. Why did the antagonist pull him in there? How is my MC going to get out? I can still showcase some of his more heroic aspects by how he resolves the conflict, and I've created a question that will (hopefully) keep readers reading.
And on a side note, I encourage everyone to go on Goodreads, start following the heavy readers and reviewers and take note on what they say. They probably read almost as much as editors and agents, and I've gotten many ideas on what to do and what not to do by what they find unoriginal or overdone.