Gross profit is what you get if you are a big enough actor or producer to demand this. You get a percentage of profit based on the gross of the film - how much it makes before any costs. Therefore, you are guaranteed to get paid since the studio can’t hide anything. Very few people can demand gross points. But if you have them and your film hits big, you make millions. In fact, this can be so profitable you will sometimes see big stars forgo any salary at all except union minimum just to get these points.
Unfortunately, Mr. Prowse relates what is almost universally the case with a net profit clause. Studios almost never pay on this clause, as they claim nearly any and every expense possible to keep the film from showing any actual profit. Very few films have ever shown a net profit on the books.
How do they do this? Well first, imagine that George Lucas decided to go to New York tomorrow to talk about showing Return of the Jedi in 3D. And he stayed at the Ritz Carlton, ordered sushi at 3 a.m. from room service and used the hotel phone to call Bahrain to make prank calls.
Well, 26 years after the release of the film, the accountants at Lucasfilm are going to charge $86,000 to the costs of Return of the Jedi. I am NOT joking. This is what they do. If George Lucas utters the words Star Wars and he’s spending money, they’re putting it on the red line for one of those films.
On the flipside, Mr. Prowse would be wise to use the FORCE, aka a lawyer, to get Lucasfilm to cough up. You see, as you can tell by the above, the accounting is utter ********. And on a film like Return of the Jedi, Lucasfilm would be extremely reluctant to open its books in open court. Extremely reluctant because of how incredibly embarrassed studios have been in the past when they have made the mistake of doing this. Plus, the ensuing publicity would be embarrassing to Lucasfilm. Can you imagine the hedlines if Darth Vader sued George Lucas?
In short, if you have net participation on a film that has grossed hundreds of millions, you may get some dough, but you’ll have to sue to get it.