Choose one of the primary characters of your story. Map out his actions, and his changing motivations, over the course of the events in your storyline, assigning each to either a date, or to a period of time in his life. For example:
Age 27: John meets Karen, the love of his life
Age 28: Karen is lost and presumed dead in a mountain-climbing accident
Age 30: John marries Cheryl, believing himself to be over Karen
Age 42: Karen returns to John's hometown, after being nursed back to health by Tibetan monks
Extend the timeline backwards, exploring the events in John's life prior to the beginning of the story. This fleshes out the character, providing you with additional details about who he is, and how he thinks. Many of these ideas will never see print, but they will still inform your writing, and may provide you with a crucial plot point.
Age 16: John's high-school girlfriend leaves him for Joseph, his best friend
Age 18: John attends the University of Colorado
Age 21: John takes a year to study abroad
Already there may be an intersection: John went to college in a mountainous region. Perhaps he was a skier, or maybe he intensely disliked wealthy tourists. This may relate later to his reaction to Karen's mountain climbing passion, or might intensify his feelings of loss.
Create another timeline for the next major character. Explore Karen's life before she met John. John fell for her immediately; perhaps it was unrequited at first. Her history, or his, may explain why -- a skiing accident may have made him too timid for her outgoing personality.
Mesh the timelines together. Pay close attention to how these timelines interact, and do not introduce anomalies. If you wish Karen to be a ravishing woman in her late twenties when she returns, after a fourteen-year absence, then you have accidentally described John as Lolita's Humbert Humbert. Shorten the time she was missing, or age the character accordingly.
Add additional characters to the timeline, until you have a complex interacting character study demonstrating how your protagonists come into and leave each other's lives.
Add key events and turning points from your plot to these timelines, and ask yourself how each one would affect each character. If you need an event to have a greater emotional impact, edit a character's back story to make it more affecting. Rich character histories contribute to the reality of your story, and motivate the reader to care about the characters and what happens to them.