We just had a presentation of collaborative writing. I am a fan of collaborative writing -- even though I have been sullied on business partnerships in general. While I will firmly suggest I will not partner in business again (see Buddha, I learned my lesson), I will eagerly partner in writing books or screenplays again. When you have the right collaboration, you will find it enhances the process. You can brainstorm and bounce ideas around with someone else and get feedback from a vested partner, who is much less inclined to indulge you since he/she has a stake in its success. You can be each other's fan and support club to finish the project. You have someone to enjoy the process with too.
On the downside, a bad partnership or collaboration can have horrific and stressful results. So, make sure you have an agreed upon vision and compatible writing style. Do watch out for a collaboration where one partner takes advantage of the other's goodwill and generosity. Signs of abuse are the collaborator not doing her fair share of work, not contributing an equal amount of financing to the project, or going behind your back and making agreements about your project without your involvement. These signs are project and relationship killers.
The taking-advantage problem probably happens most often. So watch out for potential collaborators who might have a sense of entitlement on your time, money and resources. People like this generally show their stripes pretty quickly. What I've learned is that if a collaboration immediately starts to go out of balance, nip it. Do not keeping shoveling your generosity at it, because people who feel entitled will keep right on taking.
And do not get in the mode where you feel like you should because you think you have more to give. If the results of your collaboration will be a 50/50 split, the investment should be 50/50 regardless of any emotional blackmail the offending collaborator might pull. So just keep it even regardless, and it will set a precedence to keep the relationship from going upside down. Because how you set up and behave in the early part of the collaboration will set the tone for the future of the relationship.
The biggest mistake I've made is "giving away the farm" to someone who I should never have done so with in the first place. Would the outcome have been different had I never given so much of my resources and money and not received reciprocity? I don't know. But I do know that I wouldn't be standing looking at a finished product that because of this kind of bad blood, I will not put the right amount of effort into selling, because that person is still attached. Ultimately, though, this is my bad for not being more cautious and demanding equality. So learn from my mistakes.