This is always a pickle. Suing a studio for breach of contract and/or non-payment. Especially one you just worked for (as Cassavetes did when he directed My Sister's Keeper). The project in question is apparently (we're not saying New Line didn't pay Cassavetes, because studios never do that kind of thing. Ever.) some orphan elephant drama called Peacable Kingdom that Cassavetes was attached to back in March. New Line dropped him like a bag of cement, and according to Cassavetes, without compensating him for a rewrite he did. Also, Cassavetes claims he passed on several projects while he was working on Kingdom, and once he was let go, those opportunities were no longer on the table.
Now, a studio has never failed to pay me for work I have done. However (big however), they have taken their sweet time about getting me the money. I once sold a TV pilot in July, and didn't get paid the full balance of the deal until February. But, they paid me.
A lot of times what writer's face is the dreaded, "Oh, those were producer's drafts. The studio didn't request the revision, so we're not paying you for those drafts. You need to get the money from the producer." Oh, okay, because producers are always just walking around shitting money.
I'm not saying that's what happened in this case, but that's what happens. A lot. And, most of the time you just have to suck it up.
As far as Cassavetes wanting compensation for passing on a few projects--he's probably SOL on that one. People take jobs all the time, or leave them, and miss out on things because of it. I can't imagine anyone wanting to give him a dime for that--unless he was fired without cause.
That'll be a much different story.