I didn't so much agree to collaborate (retrospectively) on my husband's novel so much as I failed to express just how bad of an idea that was.
Long story longer than necessary: I used to write with my two oldest friends. What I discovered through our process of occasionally starting something and frequently getting nowhere, was that it doesn't work for me. As the old saying goes, "I can do bad on my own." I don't actually need help not finishing a work.
But I was the one who roped X into writing. We met through a role playing game (thank you, White Wolf ), and it was his story telling ability as much as any other attribute that won my heart. It was only a matter of time before I started to encourage him to share all those fabulous stories with a wider audience. He eventually gave in, joining the writer's group hosted by Kelley Armstrong and participating in NaNoWriMo. A fellow writer so liked his work that she put in a good word with her agent, who in turn liked the first three chapters so much that she asked for the whole thing.
That's where his path to success met a road block. This agent, who has worked with some major names in speculative fiction, loved his story as well. She offered to represent him IF--yeah, the if is the problem. The middle was weak. He could deal with that. She wanted it changed from 1st person to 3rd, which is time consuming. The more time that went by, the less he worked on it, until he turned to she-who-kept-correcting-his-grammar.
It's been both difficult and rewarding. Then, the other day, my insensitivity--or rather my failure to realize what his insensitivity might be masking--created an explusion in both the writing partnership and the marriage. Working it through as a couple helped us to work it out as a writing team, but this got me thinking of all the would-be novelists I'd heard of who didn't make it as co-authors. And that got me looking for advice from published novelists who had.
Kathleen Baldwin tells us that the first lesson in successfully collaborating is picking the right partner. Follow the link on her page to see what her partner says.
This rather rambling article includes a few words on how the married team of Agnes and Egerton Castle do it.
Holly Lisle gives the good, the bad, and the ugly--ok, mostly the bad and the ugly--in this great How-to on collaborating.
If I find any more links that might be helpful, I'll be sure to update this. Meanwhile, I've got some fiction calling my name.