The point in the story in which this problem is introduced is called the inciting incident. It’s here where, as writer Robert McKee puts it, the protagonist’s life is thrown out of balance and, hopefully, where the audience begins to want to see this balance restored. It’s not just a problem, but the problem from which all the other problems follow on. In 'Jaws' (1975), for example, this comes at the beginning of the film, when Chrissie Watkins is attacked by the eponymous great white while swimming at night. In the scene immediately following this, we're introduced to Chief Martin Brody (Roy Scheider) as he inspects the young woman's body and argues with the complacent Mayor Vaughn. At that point we know, almost as soon as we see him, that he will have to kill the shark to resolve the plot. The immediacy of this inciting incident fits perfectly into the film’s structure, casting it from the beginning as a tense, contested, harrowing challenge for both the characters and the audience.
In the inciting incident in 'Jaws', Chrissie Watkins is attacked, at once frightening us and setting the plot in motion.
Hitchcock the master storyteller
By contrast, the inciting incident in 'North By Northwest' (1959) is much more unpredictable. (You knew what to expect from a movie called Jaws with an enormous shark on the release poster.) Roger Thornhill (Cary Grant) makes the mistake of catching the attention of the busboy in the bar in which he’s attending a meeting, and is then kidnapped by two vaguely European (and therefore sinister) goons in bad suits. It’s the kidnapping that’s the inciting incident, but it comes as just as much of a surprise to us as it does to Thornhill - it’s weird and unusual, and nobody can quite grasp how serious it is until too late.
Cary Grant is kidnapped, setting off the chain of events that throw his character’s life out of balance. Once they’re in the car, we can also see a good example of rear projection!
Thornhill has, up to this point, been the master of his world: he doles out orders to his secretary, has important meetings in exclusive restaurants, and generally seems to think himself cock-of-the-walk until he meets the two thugs who won’t entertain the idea that he’s not who they think he is.
After he’s picked up by the stone-faced pair in the Cadillac and spirited away to a suburban mansion, he gradually begins to lose the control he thinks he’s had and the audience realises it right along with him, as he’s interrogated by James Mason’s Phillip Vandamm, then drugged, nearly dropped off a cliff, and then left still more mystified by his discovery of an espionage ring.