"Jane was a shy girl. She never talked to anyone unless they spoke to her first. Maybe this was because she wasn't very pretty, wore thick glasses, and stood taller than most boys."
Seems like an okay description of Jane, right? Maybe if you were describing her to a friend in conversation or mentioning her in a letter. But for your novel or story? No. It's boring. At least if you wrote your character descriptions this way all of the time, it would be. So, what can you do about it? You definitely don't want your writing to be boring. What publisher would want to buy that? Come to think of it, what reader would want to read it?
Let's try this again with using flashback as a tool.
"Jane walked into the reunion, her palms sweating. She could see one of her classmates waving to her. Oh, God. Why did I come? She thought of turning around. This always seemed to happen. It never changed.
Her mother had taken her by the hand right up to the front door the first day of school. After some persuasion and a gentle push, Jane walked into the room and sat in the back row. All the kids were laughing and chattering. Jane kept fidgeting with her glasses. They felt heavy and awkward on her nose. One boy sitting next to her turned to stare.
'Your eyes look funny with those on. All big and goofy.'
Jane opened her notebook, brought it close to her face and studied the inside.
'What's a matter? You deaf?'
Jane popped out of the chair and scrambled over to another desk.
'Wow! You must be ten feet tall!'
Now, everyone was staring. Tears welled up in her eyes. Jane ran out of the room and out of the building.
'Jane Goodman! It's so good to see you after all these years. And you look great.'
Jane turned to see a face she would know anywhere. Even if there were a few creases around the eyes and grey in the hair. She smiled and nodded. Maybe this won't be so awful after all, she thought."
Now, if you will notice, the flashback to Jane's first day at school helps the character to come alive. This way, the reader begins to "know" Jane, become familiar with her. And in the long run, the reader will become "invested" in her character, care about what happens to her. When readers have this kind of empathy for the character(s) of a book, they want to keep on reading, they have to know what will happen.
Flashback is a wonderful tool, but there are a couple of warnings that go along with using it.
1) Don't announce the flashback with phrases like "it brought me back to" or "I remember the time" or even using ellipses ... Changing the scene with a certain look, (for example in mine the setting is the first day of school) and introducing it with the past perfect verb tense (in mine I used, "Her mother had taken her") is one example of how this works.
2) Don't announce the return from flashback with phrases like "that was in the past" or "I was brought out of my daydreaming". Just changing the setting with a few words or even a bit of dialog will work. (In mine I used: "Jane Goodman! It's so good to see you...")
3) While you are new to using flashback, be careful of the length. The longer the flashback, the harder it may be to transition in and out. Many use a separate line of asterisks **** or white space.
Overall, giving your writing a bit of variety with flashbacks to describe characters or to explain needed information without literally "explaining", i.e. telling instead of showing, will make your writing pop and sizzle off the pages, make your readers cry for more. And that's what we all want, don't we?