There's nothing worse than a bad collaboration.
I have had them. In one a collaborator tried to steal the material, making it his own; thinking it was his own. And this was after a contract had been signed and I had done 80% of the work.
There went the trust and here came the feeling that I never wanted to see, much less be in the same hemisphere, with this guy again. But we had a contract, that same contract that bound us together in the first place and now had me shouting: LEMME OUT!
Rule 1
Always sign a contract first, no matter what. Figure out how much responsibility each of you will contribute, and go for it. What if you didn't have a contract! Think of the hell you might have then. I had one of those in a collaboration with a major A-list writer, my best friend. I thought, to my later consternation, that we didn't need one. We were best friends. In fact, it never occurred to me. At one point he offered to give me an interest-free loan as my part of the bargain. That's where the friendship ended.
The script, which studios were waiting for because it was good and they had had a sneak peek, went nowhere because my agent and his agent couldn't work it out. Thinking back maybe I should have taken the scraps just to get it made.
But ALWAYS have a contract. No matter what.
Be grateful. Work it out. Explain that the contract represents time spent, yes, but also time working in the business counts, and that you actually THINK about the work when you're not at the computer or yellow pad.
Rule 2
Contracts have nothing to do with (and everything to do with) TRUST: "I have done more work on story than you have, so I want at least half." The Writers Guild of America breaks down the writing process loosely as this: $25% for the story; 75% for the screenplay. Story is compiling, writing is putting the movie on the page. They overlap but that's the breakdown.
I had a collaborator once who kept insisting that everything was hers because she had spent all those years compiling and thinking about the characters, while I spent my years writing and having produced movies and TV and books. She didn't know how to put the movie on the page. She thought she did.
She became PROPRIETARY over the work. It's mine, she'd scream. Mine! She'd throw a tantrum. That's how she got a lot of what she had. Tantrum Perks, she called them.
I tried to explain. I threw my own more amateurish tantrum, and then finally, I said, no. NO is a stopper. GETTING TO NO is good. But only half way there.
Rule 3
You want to Get to Yes. To agree on item after item, so that you're thinking in tone, story, reversals, etc. And still thinking individually. "Yes, I agree that you have done work," you say to your partner but ... or: "Yes, I agree that you have done work. Okay, I'll give you another five points."
Be careful here, though. That old adage: give an inch, they'll expect a mile. Or the collaborator will be grateful and work harder. In that case, count your blessings.
No Ego is the mantra. The only objective is a better script and then the best script possible. If things are deadlocked, there always should be one writer with final decision-making abilities.
Many if not most scripts produced these days are written by teams. It makes sense. All those decisions can be made and discarded quickly. One writer often is better with dialogue, another with structure. One is good inside the scenes, the other excels in concept. One is driven by fear, the other by overconfidence.
I recommend, for romance, working with a lover. For writing a good script, I would recommend against it.
Every day before you start writing reread what you've written from the beginning. It will lock you into place and you won't waste time.
And always ask yourself: is this suggestion or that suggestion based on my ego wanting its own way or my desire to produce the very best script?
Don't rewrite each other unless you first agree on it.
Before choosing a collaborator always ask to see samples of the other person's work. Maybe you have a genius in your midst who has completion anxiety or emotional problems. Listen to your gut. It's usually right.
Hold your temper back and watch out for RESENTMENTS. They will surely kill you, and the project.