Getting In The Character's Head
... Characters are not lifeless mannequins that move through stories from plot point to plot point. If you want an audience to love your characters and feel anything for them, then on some level the audience has to perceive every character as a living, breathing entity that thinks and makes choices that are rational. There's no way around it. Character and plot are not two separate things…they are interwoven. ...
Characters have to act ... and react ... in ways audiences find believable. It undercuts the feature being sold if people sitting out in the dark go "whaa?" when a character behaves in a strange way.
Like for instance, Copper the hunting dog in Disney's The Fox and the Hound. The animal goes into a rage over a non-fatal accident to another dog, because the plot needs Copper to get angry and swear vengeance on Tod the Fox, whom he blames.
The dog's anger felt forced to a lot of Disney story artists at the time, including me, but the directors of the film overruled the story crew. The picture was profitable, but the story pivot was awkward because the dog's reaction didn't seem right.
There are other pictures that strike me much the same way, but since I was an employee at Feature Animation at the time, the TFAH example is the one I use to underscore Mr. Kennedy's point up above.