Originally posted on my blog but I wanted to share with everyone here.
A character's past influences their decision in the present. Your character may have witnessed his mother killed when he was younger and plans to find the murderer at any cost. This past element will drive all his actions in the present. Every decision he makes is to find this person. Nothing will stand in his way. This character is meeting his past head on.
Last night when I wrote in my novel's journal, I figured out my hero's past element which will take most of the series for him to overcome. My hero is running away from his past because he does not want to make the same mistake. Since I made this discovery, the first few chapter need to be rewritten. The actions my character has taken does not reflect this past element at the beginning.
The conflict will come when the situation for the character puts them against his present course. Let us go back to the first example and add this scene in. The murderer sets a home on fire trapping those inside. The main character must make the decision to keep going after the person who killed his mother or save the people trapped inside. Either choice will weight heavy for the character. And it will influence any future decision he makes.
You should keep these thoughts in mind even with secondary characters, especially if they have any impact on the main plot and/or sub-plots. You do not need to let the reader know their situation, but you need to understand to weave their actions right.
The character's present action must match with their past. Even as the story moves forward, anything that happens during the present in the novel will turn into the past for the character as you continue to write. The past will carry on influencing the present throughout your novel. Future conflicts will be created for your characters based on their present action. And conflict is what drives your story.