First of all I don’t watch too many movies. And nowadays, watching movies has become even harder as I try to double-guess the plot.
Fundamentally, commercial movies like to tell stories that are spectacular, which means they should consist of low-probability events. Think of defusing bombs when there is 1 second left on the timer, for example, or the heroine’s flight getting delayed just so that the hero can catch her at the airport.
Now, the entire plot of the movie cannot consist of such low-probability events, for that will make the movie extremely incredulous, and people won’t like it. Moreover, a few minutes into such a movie, the happenings won’t be low probability any more.
So the key is to intersperse high-probability events with low-probability events so that the viewer’s attention is maintained. There are many ways to do this, but as Kurt Vonnegut once wrote (in his masters thesis, no less), there are a few basic shapes that stories take. These shapes are popular methods in which high and low-probability events get interspersed so that the movie will be interesting.
So once you understand that there are certain “shapes” that stories take, you can try and guess how a movie’s plot will unfold. You make a mental note of the possible low-probability events that could happen, and with some practice, you will know how the movie will play out.
In an action movie, for example, there is a good chance that one (or more) of the “good guys” dies at the end. Usually (but not always), it is not the hero. Analysing the other characters in his entourage, it shouldn’t be normally hard to guess who will bite the dust. And when the event inevitably happens, it’s not surprising to you any more!
Similarly, in a romantic movie, unless you know that the movie belongs to a particular “type”, you know that the guy will get the girl at the end of the movie. And once you can guess that, it is not hard to guess what improbable events the movie will comprise of.
Finally, based on some of the action movies I’ve watched recently (not many, mind you, so there is a clear small samples bias here), most of their plots can be explained by one simple concept. Rather than spelling it in words, I’ll let you watch this scene from The Good, The Bad and The Ugly.