
Entertain yourself.

As you delve into the five-page summary of the last twelve years of your character's life, ask yourself: is this interesting?  Be honest.  Do you like reading summaries?  Or do you prefer to meet characters as people, watch them grow, let them breathe, listen to them talk and walk along the path with them awhile?  Give your readers the same chance to know your characters the way you do.  You don't need the brand of their breakfast cereals, but one wistful flashback to a perfect (or less so) day will do much more to give your readers a feel for the character than reeling off his/her resume and favorite pastimes.

The converse of this is the question, "Is this important?"  It might be important to you, it might be important to the character, but as much as it's good that you know the details, does it add anything to the story?  Sometimes the answer is going to be no, and you need to hold back.  Don't tell the reader what the minor character had for breakfast the other day, not unless it's relevant.  Bring forward details to define the character and fix him/her in your reader's minds: maybe a single physical feature, or a particular talent, even one lost dream.  Reveal the rest with dialogue, not necessarily having the character tell someone else everything, but instead in the words s/he chooses to use, the things s/he changes the subject about, and so on.

Flashbacks are optional.  Dr. Merlin believes the pensieve from the Potter books was a fantastic way of working in flashbacks for a series told almost exclusively from a youngster's point of view; they would have been bizarrely shoehorned in otherwise.  Sometimes a flashback is a writer's best friend.  Flashbacks allow a writer to show a scene rather than merely describe it secondhand.

On the other hand, a well-written stretch of dialogue as a character is relating a story to someone else can give important information, not necessarily about the story itself, but about the character speaking: what s/he thinks is important in the tale, who features most prominently (and how this reflects back on the speaker's personal interest in the story being told), and what s/he leaves out due to the audience.  It's a fine line to walk.  If you find yourself using the speaker as an exposition-dump, stop.  Back up.  Try again.  Rewrites never killed anyone.

At every point, ask yourself how the story you're telling is going to affect your readers.  Do your readers know the characters in question well enough to care about what happens to them?  In general, if the character is from canon, the answer is probably "Yes."  Much of your legwork has been done for you then, and you can focus on how the events of the story affect the character, assured that your audience is already buckled in for the ride. 