In terms of creating interesting and viable characters, the first thing collaborators need to decide is how many central characters there are in the story. There are tracts on creative writing that say that any story has only one central character. It’s an interesting notion but an unhelpful one. Your novel will most likely feature maybe a dozen named characters but which are the central characters? Your central characters are the ones which the action focuses on, whose highs and lows we will care about most. They’re the ones who, if your story was a Hollywood blockbuster, would have the actors’ names on the movie poster. Basically, the central characters are the ones without which the story just doesn’t function.
For the collaborative writing team, clearly defining these central characters is vital. Your world, like a board for a game, may be fully laid out and its rules understood by all the players but it is the characters who will be moving around in that world. The characters are your game pieces and you need to know under what rules they operate.
Characters have rules? Of course they do. There are things that your characters will never do. There are things that they will do at every opportunity. There are things that they will do even though it is against their best interests.
Once again, it is genre that often decides that nature of your central characters. Your chick-lit romance is unlikely to feature a sadistic serial killer. Your spy thriller probably doesn’t have a happy-go-lucky granny as its central character. If you have already chosen your genre then a lot of character-defining decisions have possibly been made for you.
Genre provides us with a wealth of stock characters, tropes and clichés. Whether its the femme fatale, the gentleman thief, the pale and beautiful vampire, the tart with a heart, or the jaded veteran cop, stock characters speak to us all and are almost inextricably linked to their associated genres. Stock characters (suitably tweaked) can be a great thing up to a point. They are easily understood, relatively easy to write for and tend to create their own narrative.  They are also dangerously seductive to collaborative writers.
If two collaborators decide to write a novel about a down-on-his-luck private detective they may have lots of ideas about his character but the ideas they both come up with will come straight from the book of lazy clichés. Their detective will have a seedy office, smoke too many cigarettes, have an estranged wife/girlfriend/daughter, be able to take a beating, and possess a strong personal moral code (which probably caused him to get kicked out of the corrupt LAPD years ago). Sure, this is all fine but there are lots of opportunities missed here. Did we assume that our private dick was white and American? Did we assume that he was a drinker with no real interests beyond his work? Did we assume that he was a he?
Collaborative writers, because of their shared cultural experiences, have to work doubly hard to get away from the clichés of that shared culture. Heide and I used the following game to develop the characters for our novel, Clovenhoof.
Creating Characters – The card game
Take a packet of index cards AKA file cards (or postcards or squares of paper, whatever). How many? At least thirty but the more the better.
On each one write down a character trait. They can be emotional adjectives (manic, whimsical), social traits (outgoing, loutish, a good listener) or moral/political viewpoints (closet racist, republican, utilitarian). They could be personal likes or dislikes (hates cats, loves sushi, indifferent to fashion), personal habits (picks nose in public, skims stones, prays nightly) or telling physical traits (toned muscles, unwashed clothes, fake tan).  Whatever comes to mind, write it down. It doesn’t matter if it seems stupid or frivolous. You can choose to ignore it later or find a surprising use for it.
If you’re writing a genre novel then throw in every genre cliché you can think of (smoker, trilby-wearer, estranged wife, sleeps with gun under pillow). At the same time, create some genuine genre breakers (salsa dancer, Buddhist, optimistic).
In terms of genre-breaking and to add some general spice to the mix, get some friends and loved ones to come up with character traits. If they don’t know what your novel’s about then all the better. A bit of randomness may lead to more original characters.
Creating one character
Spread all the cards out on a surface (or a selection of them if you have loads). The collaborators, having already agreed the function or purpose of the character in their narrative, look at the cards.
In turn order, each collaborator removes a card that they think is not appropriate for the character. You will need to decide up front if collaborators can disagree with the selection or over-rule each other (each has a one-use veto, for example) or if any decision to remove a card is final and unarguable.
Keep removing cards until you have a previously agreed number of cards left (10-12 is a reasonable number) or until the collaborators are happy with the remaining cards. What remains is the outline of the central character.
Creating a number of characters equal to the number of collaborators
Spread all the cards out on a surface. Each collaborator is randomly assigned one of the characters.
In turn order, each collaborator selects a card that they believe describes their character. Once a card has been selected for one character it cannot be selected for another. In this way, genre-specific traits are limited to single individuals. For example, in your noir thriller, only one character is a chain smoker, only one has an estranged spouse, only one is a bitter and disillusioned ex-cop.
Keep removing cards until each collaborator holds a character-defining selection or until all the cards have gone.

Creating more characters than there are collaborators
Each collaborator is randomly assigned one of the characters. All the cards are dealt out at random.
Collaborators discard cards that they don’t feel suit their character. They do this until they are happy with the selection they have left.
The discard pile is examined. Collaborators might wish to take for themselves a card that someone else has discarded. Whatever happens, when the collaborators are happy, the remaining cards in the discard pile are used to create remaining characters.
Example:
Leslie and Hilary are writing a chase thriller set in contemporary England. They have already settled on three central characters: the MI6 agent working to expose a dark government cover-up, the innocent everyman character who accidentally becomes embroiled in the conspiracy and ends up on the run, and the police officer who is working the case from another angle and becomes the everyman’s one ally.
Leslie and Hilary have a bank of 90 cards (most of them left over from a previous collaborative exercise) and divide them into two roughly equal piles.
Leslie, creating the MI6 agent, has cards such as ‘aerobics fanatic’, ‘over-bearing boy/girlfriend’, ‘amoral’ and ‘Tory voter’. Leslie decides to keep ‘amoral’ and ‘Tory voter’ but discards the other two.
Hilary, creating the police officers has cards such as ‘Brummie’, ‘keen sailor’, ‘ex-Muslim’ and ‘fear of commitment’. Hilary discards ‘keen sailor’ and ‘fear of commitment’ but also picks up Leslie’s discarded ‘aerobics fanatic’.
The cards left in the discard pile, such as ‘over-bearing boy/girlfriend’, ‘keen sailor’ and ‘fear of commitment’ become the basis of the third character, the innocent everyman. Leslie and Hilary go on to discuss all the cards in the discard pile, removing those that don’t suit the character they wish to create.
However we use this game or others to help create characters, we must remember that such games serve us and our stories. If you don’t like the results, change the method and try again.