She threw a molotov at the mouth of the beast. Setting up pivotal scenes and moments.

Spoiler alert. At the end of 10 Cloverfield Lane, all hell breaks loose. She decides it's time to leave this prison of a doomsday shelter and that her captor is probably a over paranoid rapist who uses doomsday events as a cover for imprisoning his victims. She kicks over a barrel of sulfuric acid at her captor and he trips over it to take a drink, she then hops over, takes her home made radiation suit and a bottle of compressed air and heads for the air vent. It's all a really intense, anxious scene.

Is she going to get out? How is she going to get out? Will he come after? When she enters the air vent, the audience doesn't say it, nor does it need to be said in the movie, but it's something like 'do you remember this air vent, there's a way out this way?' My point is that what if they never introduced the air vent in the beginning? What if the exhaust fan didn't break down and she wasn't the only one that could fit in and repair it from the inside? Well, if they didn't set that up, the audience may have been brought out of their realm of movie rules and thought 'hey where did this come from? Well i guess anything can happen now.'

It's much better to have a concept set up in the audiences' mind before adding it in to the movie especially at a pivotal point in a movie. You don't want to lose your audience by forcing them to believe something at such a fast pace. If Harry Potter was a normal kid the whole movie and all of a sudden at the end of the movie fighting he-who-shall-not-be-named, whips out a wand and blows him up with a wizardly phrase, well the audience will think 'when was he able to do that??' And then wizardry would have been an afterthought for Harry Potter and Rowling probably wouldn't have become rich.

 It's not because you and me and other movie/netflix-goers are so dumb that it needs to be explained. It's the fact that we ARE smart. It needs to be setup accordingly and if a concept like 'oh, i can use this air vent as another way to get out because it's where i saw a hatch and just a single padlock' is thrown in out of nowhere, the audience is going to think, 'okay wtf, what else are you going to do now... make her happen to find a box full of medical supplies and guns so she can survive?' The writer or director or producer clearly didn't think it through and sort of just squeezed it in there. (Usually people don't say this but they can feel it when they watch it)

Some small things don't need to be setup, some do. It's experience from seeing what works and what doesn't in hundreds of movies that will give clues. If what you're throwing in makes big changes in the character's journey or situation, it probably needs to be setup. If it's something like, having a bottle of alcohol and matches happen to be the back seat of a pickup truck, it doesn't beg to be setup, because it still doesn't change the fact that she has to do something about being trapped in a truck as a giant alien reels it in to be chewed up as a bland auto human sandwich.

Again, it's art, not science.