One way to reveal a character's past is through flashbacks. Flashbacks do interrupt the flow of a story, sometimes testing the patience of an editor or reader. Good writers use flashbacks wisely.
Donald Maas says to write the flashback "in dynamic form: Structure it as an action section, complete with failure." In other words, show the encounter exactly as it took place in the past.
Here's how he recommends writing flashbacks: A present-time story cue sets up the flashback. For example, the heroine sees someone who reminds her of a past encounter. She notices his eyes, or hears a voice she'd hoped never to hear again.
Maas recommends writing two 'had' sentences to take the reader into the past. Then write the flashback in simple past tense terms. Use two 'had' sentences to signal the end of the flashback sequence and pick up the present story.
"Keep flashbacks as brief as possible; trim it to its most pertinent action. If it must run on for more than a couple of pages, split it into two or more flashbacks, bringing us back to the present story in between."
Most importantly, insert the flashback in a relatively quiet moment of the story. Interrupting a dramatic or high action scene with a flashback is not advisable.