Spending a lot of time imagining your character’s back stories and previous lives is time you’ve wasted.
In my honest opinion.
This flies in the face of so much you read elsewhere, but I’ve always found it to be so.
It’s all very well creating all these wonderful people to the nth detail- the minute the story starts flying, then all that detail goes out the window and a whole other level of invention takes over, based on that peculiar state of being when a character (the bit you have probably imagined quite clearly, down to the color of their underpants on Fridays) starts making choices (the bit you probably haven’t, because you’ve only started thinking about these particular, life defining choices now you are actually writing the story).
Look, no need to get bogged down. The way I do it is far far easier.
Think about any fascinating people around you.
The one thing they all exhibit is contradictions.
Someone may be utterly arrogant and aggressive, yet randomly kind. Someone may be absolutely obsessive about the neatness of their appearance and their house, yet utterly messy and random in their love life. Etc.
The trick is to get a couple of these big contradictions into your character. Three should do it for a main character - and then all you need to do is to make sure the story gives lots of chance for these contradictions to affect the course of the action.
When I go into developing the story, that's pretty well all I have in my head as I start to work out the beats.
Oh, there is one more thing to add.
The main character must change in his/her journey.
This is also known as giving a character an 'arc'. If you're writing a movie, I think it's pretty well essential that your character has an arc. absolutely! In fact I'd say that without an arc then it's arguable that you don't really have a movie - or at least you're going to have to work ten times as hard to hold the audience, and your audience is going to be probably rather more limited.
(Yeah, I know James Bond doesn't change much, but look at what they have to throw at the screen to keep you interested.)
The exception to this is you're writing an episode of a classic tv series then you can get away with a lot less – you can't take characters too far, as you have you come back and find them roughly in the same place for next week's episode. You need some light change by the end, but it's got to be forgotten by the time the next week's ep starts.
So what is this 'arc'?
There's a lot of complicated theory about - all to do with flipping emotional states and "negations of negations". That's all very well - and it does make terrific sense when you really get into it, but to start out you don't need it.
To put it simply, by the end of your movie the character wants to be more or less the opposite of what they were when the story started.
If at the beginning of the story they are a supremely successful lawyer making millions a case, dressed in Gucci, driving a top BWM, with a sleazy set of morals, then by the end of the story it would probably have been a smart idea to consider showing how they have risked everything, lost everything, and are finally broke, bankrupt, wearing charity shop clothes - but with a keen moral purity.
If at the start of the story your character is an idealist opposed to the filthy way his mafia family make money, then by the end it's going to have been a great story if you have shown us how they have become the biggest, most ruthless gangster at the heart of the family business.
Those are two very clear oppositions, from two terrific movies. See what other changes you can identify.
This is a very quick summary of the method I use to create characters in my own writing. If you're interested in finding out more then I go into far more detail in my downloadable package The Screenwriting Goldmine Guide To Writing a Screenplay. I’ve written a lot of scripts for a lot of TV shows, and in the Goldmine I go into full detail, and give you a couple more neat templates that show you how to structure your thoughts on character in the most economical way, so you don't waste days doing work that is, in the end, a lot of red herring! Download it here.