It is quite common to offer the cast and crew a share in a film rather than paying their fees. It helps producers minimise the need for cash flow. Many crew members do turn this offer down but some, on the other hand, have made huge profits on the back of this.
Steven Spielberg was working on JAWS. It went heavily over budget but he needed more money to finish the film so, he agreed to forfeit his director’s fee in exchange for a percentage of the gross profits. The studio didn’t expect the movie to be a huge blockbuster, so they jumped on the offer. Jaws became a high earner and so did Steven Spielberg. After all those years, he is still collecting his cheques.
Another big example is Harrison Ford. He made huge profits from Star Wars. Now it is a norm for actors and directors to ask for a profit share.
James Cameron made over $350 million from a similar deal for doing Avatar.
King Kong gave Peter Jackson 20% of the gross against his 20 million dollar salary.
So it can be safely said that accepting a percentage against a fixed fee can be a great deal but the question is, do you have faith in your product?