There's no getting away from it, at some point, normally very near the start of your project, you're going to have to come up with a character list, and if you write for television, stage or screen you're most likely going to have to submit that character list as part of a proposal before you've even written the story. So how can you possibly know your characters before you've even got to know them? Here's how.
Be specific not prescriptive
Being vague about a character may allow you the space to develop them later, but you'll be doing it without the commission. Saying a character is 'good' or 'bad' isn't going to cut it. Like all strong writing you need to find the specific word - the telling detail - that really captures the essence of that character. What is it about her that is good? Is she honourable, trustworthy, loyal? If you get this right at an early stage you won't be restricting your story but providing germinal seeds of character that will blossom into fully formed people informing and driving your plot.
Understand the relationship between character and characterisation
Characterisation is the collection of details that make up a character: hair, clothes, job, etc. whilst character is what the character do or are i.e. the nature of that person. Character can be revealed through characterisation, although it's most powerfully revealed in action. Don't obsess about characterisation details if what you're really trying to grasp is essential character traits and likewise, don't struggle for core character if the only thing that's important for the story is the fact that the character has pink hair.
Discover the essentials
This is the key to being able to define your characters without making a decision you'll later regret. If it's important that the character has worked hard all their lives but you're not sure what job they do, don't make one up for the sake of it – that's something you can leave for later – just specify the work ethic. On the other hand, if the piece is called 'Death of a Dustbin Man' then you have to mention it. This is the decision you have to make – the character elements that are essential to the story you want to tell.
Make a good character flawed, a flawed character virtuous
Give a good character at least one flaw and a flawed character at least one virtue – otherwise they will be unconvincing and uninteresting. And you need to do it right from the off. It's as simple as that.
Forget the Story
Resist the temptation to start telling the story in the character descriptions – focus on the characters' goals and motivations – giving some story elements may be necessary to do this, but remember that the story belongs to the synopsis, and the characters to the character list. It's worth making this distinction even if the character list is purely for your own benefit, as it will encourage thought and focus on the characters and as a consequence make them stronger and more developed.