It seems to work because we have different areas of strength. My collaborator is a former student who has great story ideas. He writes slowly, however. I write quickly.
The collaboration started after he dropped by my office to run some story ideas by me. I thought one "high concept" story was very marketable. He went home to start writing it. A week later he returned, complaining that writing was still a slow, difficult process for him. He surprised me by asking if I was interested in collaborating with him on the script.
I thought about it and said yes for two related reasons. One, I thought his story concept was very commercial. Two, I was nearing the end of my contract with my agent and wanted to get a new one higher up the power chain of command. I thought this script might be a device to do that. So I agreed.
First, we brain-stormed the story and filled out a 3-act paradigm chart on it. Then we developed a sequence outline -- the 10 major thematic movements in the story. Next I began writing script pages and Alex continued to work out details of the story.
Six weeks later, we had a script -- thrice rewritten -- that was ready to market! And my notion of its marketability is true: so far, every agent we've pitched it to has asked to read it.
And we are in Act Three of a new collaboration in draft.
So I've learned that collaboration can be a real joy. It's fun, and when brain-storming ideas, two heads are definitely better than one. In this new project I've given Alex some scenes to draft, and he does as good a job at them as I do, I just write much faster than he does.
This is maybe my fifth or sixth experience in collaboration and the only one that has been really fun. I hope we continue to be writing partners for a long time.