I'm not negotiating any contracts just now, so it's a good time to talk about my experiences in general over the past 15 years or so.
Contract negotiation is a little dance done between a producer and your agent. Deal points are negotiated. And then a contract comes back.
For some reason, the contract the lawyer sends over almost never matches the negotiated deal points. I don't know why. I guess it's because business affairs people (in-house lawyers for producers) want to prove their worth by getting a better deal than the producer already agreed on with your agent. Do they hope that the agent and writer won't notice that the paper doesn't match the deal? Or, maybe, they hope that it will be so stressful for the writer and their agent that we'll give up some of the deal points that got us working in the first place?
Producers will often try to use your cash flow against you. They offer A up front, but then they delay the paperwork until you're well into the project. When the paperwork shows up, it's A-1, where the minus-one isn't a lower number, but a flock of situations in which they won't have to pay you A after all. They figure that once you've started working, you're not going to walk away. You don't want to look like a diva, after all. You've invested time, which costs money. And you have your creative juices flowing. So you'll just sign.
I imagine a lot of writers do find it hard to keep fighting for what may seem odd eventualities -- closing loopholes, or definitions of profits or royalty revenue that may never come. I spent 10 years as a development exec writing contracts and putting in loopholes; and I have a little money in the bank, so I'm never freaking out about my next check. So I'm a stickler for getting what I was originally offered. Sometimes I think producers have found themselves paying me more than they expected, because they were counting on bait-and-switch, and I'd rather fight than switch.
Doesn't mean I threaten to stop working. I just keep working full out, and let my agents continue the haggling.
The key is to remember that they can't really walk away. They can make you think they can, but they can't. If they're Guild, they will have to show the Guild a contract at the end of the day; they can't hire you, get you working, and then walk away. Even if they're not Guild, the thing to remember is that you actually do HAVE a contract, just not a written and executed one. Contrary to what Sam Goldwyn said ("An oral contract isn't worth the paper it's written on"), when someone offers you terms, and then you accept and start working, that constitutes a contract. (The legal term is that you are operating "in reliance". The fact that your started working constitutes acceptance of their offer.)
If you are fortunate enough to be writing a film going into production, remember that the last thing they need is to set the whole show back by however long you've been working, and then have to find a new writer who can't use any of your material
-- which, even if not perfect, almost certainly contains elements of the direction the director and producer want to take it. But even if you're not in production, it will still be awkward for them if they don't have a contract. That means they can't use your work. Any of it. No matter what they say, it is not worth it to them to be in a situation where you can sue to stop the eventual movie because they haven't bought your work. They won't be able to complete their "chain of title" without your signature on a piece of paper, and that means none of the banking will go through.
So, either because of the oral contract, or their need for an eventual chain of title, or out of fear of the Guild, they will ultimately sign. As will you, because you're not a diva and you are not trying to make anyone's life unnecessarily difficult. So if you stick it out for the terms you agreed to, you'll get them.
So I just keep smiling and working full steam ahead and letting my agent repeat, "No, that's not what we agreed on," until it sinks it.
Remember, if they are revising the terms to suit them, you can do the same. You can't raise the original numbers, but if there are aspects of your contract that you haven't negotiated yet, you are now in a position to insist on clearing up the ambiguities in your favor, rather than in the producer's favor. You can get a better net profits definition, or clarify that the number of scripts you are guaranteed do not include the original development order, or whatever.
It's crucial in negotiations to get things written down. If you or your agent gets agreement with the producer on certain terms, I think it's wise to follow it up with an email saying "Here's what I think we agreed to, please confirm." Then if there's so-called "confusion" later on the producer's part, months later, you can point to the email and say, "No, that's not what we agreed on." Even if they don't respond to the email, it's part of the record that this is what you were agreeing to.