I have a good friend who is a very well known actor who married a girl from my hometown.
He told me the trick was a percentage of actual ticket sales (not just gross, as it can be manipulated between the distributor and the theaters), plus a percentage of the gross on DVD, iTunes, Netflix, etc.
Yes, every once in a while there comes along the magical, near-mythical “first dollar gross” deal... which only a handful of actors, directors or producers ever achieve - and even then, it is usually just for a short time and only on certain kinds of projects...