The Rubber Ducky

I am going to rant about the "Rubber Ducky" theory of backstory for a bit.
The "Rubber Ducky" is Paddy Chayevsky's term for when the hero or villain, at a lull in the action, explains that he is the way he is because his mother took away his rubber ducky when he was three. It's always a nice scene, well acted, beautifully lit, with a powerfully written monolog that the writer spent days on.
The character's past may be crucial to your story. Batman is haunted by the murder of his parents by a mugger when he was small. That's why he likes to dress up in latex and beat the tar out of muggers. In "The Terminator," the hero's past, which is in the future, is the hellish future of the entire human race. It sets up the stakes for the whole movie. In movies like these, we do need to know about the hero's past. You will need to keep coming back to that past, to give it the weight it deserves. Both "Batman" and "The Terminator,"in fact, start with the hero's backstory before getting into the main story.
But if all you're trying to do is give your hero more emotional depth, for the sake of emotional depth, without integrating his backstory into your story, you are running the risk of awakening the dread Ducky.
The strongest way to create a sense of character is to give the character things to do and say on screen that give us a sense of a person. If the character's personality doesn't leap off the page, readers will feel that the character is flat. Development executives will ask to know more about the protagonist's past. You will surrender to the urge to put in a Rubber Ducky. Then if the picture becomes a go, the actors will get attached to the Rubber Ducky scene, because it shows they can Act. So the Ducky stays in the picture.