In the morning Dolores stared at herself in the mirror. Last night it had all been so different. She'd walked into the bar and seen Emilio glowering at a table in the corner, so she'd gone over to him.
'Hello, Emilio,' she'd said.
Emilio looked up. 'Dolores! What are you doing here?'
'I came in looking for Juan - have you seen him?'
'No. Have you seen Conchita?'
'Not since this morning.' Dolores licked her lips. 'How about buying me a drink?'
etc etc etc until
Dolores shook her head. Thinking about last night wouldn't make any difference. She began to slowly, sadly peel her false eyelashes off.
And this is Backstory:
In the morning Dolores stared at herself in the mirror. Last night it had all been so different, so different from West Whimpering, the place she'd grown up in. She lived there for her earliest years. West Whimpering was a sleepy little town, and the inhabitants liked it that way, but Dolores had always yearned for something better. She finally left to go to the University of Watereddown to read History, but there she'd met Alberto and ended up in this one horse town. It was just like West Whimpering, but with rancheros, she thought, peeling off her false eyelashes.
Actually, I don't feel as strongly about Backstory as I do about Flashback. It does have its place, but that's rarely in the second paragraph of a short story, which is where it often pops up in my experience of student work. And then continues in the third paragraph, and the fourth. So, my concerns about backstory concern its location within a narrative and its length, and finally its relevance.
Right. Imagine you've just met someone at a party. Does the conversation go something like this....
'So, how do you know Emilio, Dolores?'
'Oh, through Alberto. What about you?'
'Through Alberto, too. It's strange that we've not met before. Tell me about yourself.'
'There's not much to tell. I live down the road and I work in accountancy. What about you?'
'How bizarre - I'm an accountant too. Which company do you work for?'
'Really? I work for Sneak, Grabbit and Fiddle.'
'You had the big tax fraud case - how did that go?'
etc etc etc
Or like this...
'So, how do you know Emilio, Dolores?'
'Oh, through Alberto. I was born in West Whimpering which is a sleepy little town, and got away the second I could to read accountancy at the University of Watereddown which is where I first met Alberto. We became friends, then lovers and then when I graduated - after a bit of a dodgy second year when I really questioned the purpose of double entry book keeping - we got married and I ended up here. Then we divorced after two tempestuous years, and I thought I never wanted to speak to him again but after a lot of water had passed under the bridge we started to see each other again and recover our friendship, so here I am! What about you?'
'I met Alberto first in 1985 I think it must have been. Our parents went to the same church and....'
etc etc etc
I hope your real life conversations follow the same pattern as the first exchange. In real life we give out little snippets of fact about ourselves as we get to know a person, we don't give whopping dollops of biography on first meeting. So why do it in fiction? So, the correct location for a chunk of backstory is going to be quite a way into a narrative, once we've got to know the characters a bit better.
The other thing about real life is that we give out our little snippets of biography on a need-to-know basis. The chances are there is stuff about your life that your closest friends don't know about you, for the simple reason that they haven't needed to know it so far. Exactly the same with fiction. Does the reader need to know Dolores went to the University of Watereddown? If the answer is yes, then put it in. If the answer is no, leave it out.
At which point a student says, yes, but we won't understand her character unless we know about her backstory, which show her motivations for behaving as she does. This is true, but often that's not what the backstory - as written in the narrative - is telling us. The mere fact of going to the University of Watereddown won't tell us much about her deep inner motivations, as presumably lots of people went to Watereddown and they aren't behaving like Dolores. Now, if someone who the reader has been led to believe is dim suddenly reveals they have a double first from Oxford, that becomes relevant.
(I'm thinking of that scene in The Graduate when Benjamin decides to have a conversation with Mrs Robinson, who reluctantly reveals that she went to university and did a history of art degree - I think it was history of art, it's ages since I've seen the film, it could have been English - which tells us lots about her boredom with life, and that she's only after sex with Benjamin not conversation, and in the process also reveals his immaturity.)
So before you put in some backstory, check that its in the right place - does the reader need to know it to continue understanding the narrative, check on the length - long chunks are as interesting as reading CVs, and finally check on the relevance - does this fact really tell us something about the character.