Writers, both solo and collaborative, need to get to know their central characters well before writing a novel about them. And the first question is not ‘how well?’ because the writer needs to know the central characters very well. The first question is which characters do they need to know a lot about and which can be sketched out most lightly.

GK Chesterton – Playing tricks with postmen

Years ago, I watched an experiment on a BBC documentary in which an actor collapsed on a crowded city street. People walked by, ignoring the prone figure for several minutes before someone stopped to help. The experiment was repeated in a quiet market town and someone rushed over to help at once.
Proof that city folk are mean and horrible? No. It was a demonstration of the fact that in a city with millions of inhabitants, the natural human reaction is to tune out most of the people. Our brains can’t cope with all those individuals. We can’t maintain hundreds of relationships (despite what Facebook might have us believe). In this world, there are the people we know, love, like and interact with and then there are… trees. To walk through a crowded city is to walk through a forest and those people swarming around you won’t become people until you get to know them.
The same applies to novels. There are characters we need to know (and love or hate) and then there are those who only exist to serve a function. Readers invest time, memory and emotion in characters and often want a little guidance about how much they should be investing. There’s a world of difference between ‘a salesman came to the door’ and ‘Pete Flowers, the one-armed salesman came to the door’. At their most basic, characters are their function, defined by their job or role. These are characters we can forget as soon as they’ve gone. They are trees in a forest.
GK Chesterton played on this in his murder mystery The Invisible Man in which the murderer who had ‘invisibly’ gained access to the block of flats to carry out the murder was none other than the unnamed postman who made a brief appearance at the beginning of the story. It’s a trick that, as far as I can tell, has since been used ad nauseum in the CSI TV series.

If a character appears only once or only briefly they can probably go unexplored as a character. They can possibly even manage without a name (unless this postman needs distinguishing from that postman at some point). It is only your named, more central characters than need depth and the more central the character, the greater depths they must have. This is more important to collaborators than to solo writers for two reasons:
Firstly, as mentioned elsewhere, when two or mind come together it is so easy to slip into areas of common experience and, culturally, that means cliché and stock characters. If we are writing about a captain of a Victorian whaling ship, we will have to work very hard to create someone who is not a lazy mish-mash of Ahab, Bligh, Hook and Long John Silver.
Secondly, when we’re not delving into shared-experience clichés, it is again easy to ransack our personal experiences for character types. If my story features a primary school teacher, it would be easy for me to write a thinly-veiled version of Mrs Fiddler, Miss Newton or one of the other teachers I had as a child. And, lazy though this is, it’s useless to my collaborator who never met or knew those individuals.
Wherever the inspiration comes from, collaborators must share as much detail as possible about their central characters. And using cliché as writing shorthand doesn’t always work. Heide and I collaborated on a character who we both instinctively saw as a bit of a geek. But what kind of geek? I had a nerdy OCD shirt-and-tie won’t-shake-hands-with-strangers cliche in mind. Heide was picturing more of a T-shirt and jeans, junk-food-junky woman-repellent type of person. The end result, explored and discussed properly, was neither of these and better for it.
Writing character descriptions and biographies is very helpful. Biographies should include basic information such as age, gender and nationality but also explore personal history, the events that made this person who they are in your novel. If they have a job or have had a job then that needs deciding. If they have family, then the members of that family need describing. Hobbies, lifestyle, levels of education and aspirations should all be included.
Writing a character biography does not – should not – mean that all those details will wind up in the eventual novel. Creating a character biography can be a lot of work but, unlike in your mathematics exams, you do not need to show your working out. Just as most fantasy novels can work perfectly well without that Here-Be-Dragons map at the front, so can your characters get by without the reader being subjected to their CV. Your character can get through the entire novel without sharing all those personal biographical details and, even if they can’t, the reader does not need to lumbered with them up front. Such character details should be revealed when needed through character action and interaction.
Consider the analogy of a party. Imagine if I walked up to you and said:

“Hi, I’m Iain. I was born in a patch of rural England where there’s nothing to do and everyone votes Conservative. My parents have been together for over forty years. My mother spent most of the seventies and eighties raising my sister and I. My father is a retired police inspector. I was grammar school educated and went on to study the Bible at a red-brick university. As a teenager, I read far too much science fiction. As an adult, I don’t read enough. I currently work as a secondary school teacher. I eat a lot of cake.”

The above may be true, it may be a revealer of character, but as a conversation opener it is strange, creepy and burdened with far more information than is useful. However, spend some time in my company and you will encounter facets of my personality that come directly from the biographical details described above. Show don’t tell remains one of the golden rules of writing.
Character and personality are seen most clearly when individuals interact with each other. People are cocktails of emotions, prejudices and ideals and they react in unexpected ways when mixed with one another. We are all understood better when put into a social context.
For that reason, it is very useful to write test pieces for the characters in your novel. The literary equivalent of a movie screen test or a theatrical character workshop, such a test piece would not be a scene from the novel but an imaginary piece in which the characters meet and mingle and spark off one another. If you have two central characters then you may write only one test piece. Three characters would generate possibly three test pieces (A-B, B-C, C-A), four characters would create six pieces (A-B, B-C, C-D, D-A, A-C, B-D) and so on. Approached with the right attitude, it can bring forth aspects of character not yet discovered. In your pairings which character is most dominant, most decisive, most intelligent, most reasonable, most sympathetic?
Such test pieces don’t even necessarily have to involve characters who will meet in your finished novel. An invented scenario in which Sherlock Holmes meets and converses with Hannibal Lector could help the writers better understand both characters.

Despite the effort that collaborators put into understanding their central characters, it is worth considering that good stories get by on very few central characters. Most individuals in a story only exist as their function, as their job. They are surface characters and don’t need to be mentally bookmarked by the reader or sketched to deeply by the writers. He may be interesting, briefly engaging and characterful but, at the end of the day, the postman is still just a postman.