
In his book Hollywood Animal
, Joe Eszterhas explains how he wrote the movie Basic Instinct
.
I had written ... movies where men manipulated the women who loved them. I thought it would be fun to flip the dynamic: to do a movie about a man being manipulated by a woman who is brilliant, omnisexual and evil. ... The piece wrote itself. I improvised all the way through. I made no notes for myself, no outline. I simply put the things down which the characters said to me. Three weeks from the time I started thinking about it, I finished the piece.
Basic Instinct is not structured with three acts. Instead it is a narrative punctuated by reference to books: the one Catherine wrote describing the murder; the one she wrote about the boy killing his parents "to see if he could get away with it"; the one she is writing, which when finished results in her telling Nick it's all over; and the one Nick says he's writing, with its impossibly happy ending, which will guarantee his eventual demise at Catherine's hands.
There are those who claim that a film must have a three-act structure, and they often claim that Aristotle gave this "rule" to us. In fact, there were no acts as we define the term today in Greek drama, and Aristotle did not talk about acts at all because the plays he analyzed were all presented in a single continuous performance.
The three-act structure that so many people ascribe to Aristotle was, in fact, invented more than two thousand years later when Ibsen and other nineteenth-century dramatists found that their audiences—unlike the drama groupies of Periclean Athens—were unable to sit still for the entire duration of a full-length play.
A movie has to start somewhere, go somewhere, and eventually finish. As Jean-Luc Goddard famously said, A film should have a beginning, a middle and an end. But not necessarily in that order. Structure is necessary and good, but beware of approaching it from a fixed POV.

Jokes have structure, but only one rule: the punchline comes last. The rest can be told in whatever order suits you, but once the punchline appears anywhere but at the end, it stops being a joke and becomes something else. One of the best illustrations of this fact is the documentary The Aristocrats
(2005), which consists of a hundred comedians telling just one joke. They tell it in a hundred different ways, but the punchline stays the same, and it always comes at the end. (I have to warn you it should be R-rated for the language.)
I think every wannabe screenwriter should study the various theories of structure until they understand them, then they should set it all aside and focus on writing their story the best way they can. The problem with embracing three-act structure as an article of faith is that you end up being able to see only what you expect to see, rather than what's actually there, and you may find yourself artificially imposing a structure on your story that won't do it justice.