It is like asking: Did the chick come first or the egg? The difficulty is that either can create the other. The same holds true for character and plot. When the characters act, the plot changes, and the plot dictates how the characters should act. Sounds confusing? It is.
The best way to simplify it is to relax the rules. You have jotted down the outline of the plot and the characters. Your characters are Mark, his wife, daughter, prisoners and cops. Mark is the protagonist. It is through his actions that the plot changes.
Mark is running away from the law, as he has been falsely accused of murdering his wife. The cops find him and put him in prison. He becomes a victim of prison fights, as he refuses to join either gang. The prison life is dragging him into scum. He tries to escape, but fails.
If you have been showing Mark as a fighter from the very start (like Rocky), then you can’t show him trying to escape from the prison. If he is a fighter, he will fight, not escape. On the other hand, if you have shown his sensitive side to the readers (him being a caring husband and father), then he can feel disgusted by the fights and try to escape.
Here, your character is giving shape to the plot. You have to ask yourself. What would a person like him do when faced with such a situation? Don’t put yourself in his situation to find the answer. He is entirely different from you and he will not behave as you would have. The story will continue in this way if he is portrayed as a sensitive man.
Seeing no option, he joins one gang and fights. He wins every time. He pleads with the cops to let him go, as there is nobody to take care of his daughter. The cops tell him that if he promises to fight for them and win the world companionship, they may let him go. He practices hard, fights and wins.
Mark is a sensitive man, that’s why he pleads with the cops to let him go. If he is a Rocky, then he will dictate, not plead. At worse, he might negotiate. Your plot will go something like this if Mark is all toughness.
The cops put him in prison. He joins a gang and soon becomes their top fighter. Seeing the possibility of profit, the cops take him under their control and force him to fight in championships. He fights for them and wins. Before the final of the world championship, he tells the cops that he wouldn’t fight unless they let him go if he wins. Helpless, the cops promise to let him go if he wins. He fights, struggles at first, then fights like a tiger and wins the world championship.
The plot is more or less the same, but the protagonist has changed. The only thing to keep in mind is that the characters and the plot should justify each other.