There are many ways of constructing a plot. One I find useful is to consider what I call the three C's:
You can use these to develop your main plot, but they are equally useful in constructing the smaller components of your story--the chapters or scenes. This is especially true in helping you construct the hardest part of any story, the middle.
THE BIG PICTURE
You can use the three C's to come up with a logline, which also is the spine of your story. For example:
A young man obsessed with becoming a great drummer finds himself tested to the limit by a brilliant but abusive teacher. ("Whiplash")
The three C's don't always occur in the same order. In this instance, it's the young man's choice (to become a great drummer) that leads to the conflict (the harsh demands of the teacher) that leads to the consequence (being tested to the limit).
This order is typical of stories in which the protagonist sets out to achieve some kind of goal. However, there are many stories in which the protagonist initially is reactive rather than active. For instance:
A young girl finds herself trapped in a strange and threatening alternative reality with three companions. The four journey to see a wizard who can give each of them what they most want--in her case, a way to get home. ("The Wizard of Oz")
Dorothy doesn't choose to go to Oz (at least, not consciously), so her story begins with conflict when she finds herself there, threatened by the Wicked Witch. Her choice is to join her three odd companions on a search for the wizard who can help them get what they want. The consequence is the adventure they experience, and her realization that there's no place like home.
THE PARTS OF THE WHOLE
Throughout a book or movie, the protagonist continues to make choices, whether voluntarily or because she is forced to, and these choices have consequences which lead to further conflict. A scene may start with either a conflict or a choice that leads to one.
A scene can start with either a conflict or a choice that leads to one.
For example, let's say you're writing a thriller in which your protagonist's identity has been stolen. Fearful that this is leading to her being framed for a murder, she takes action to discover who is responsible.
She finds a clue that the woman impersonating her is going to be at a certain restaurant for dinner. She decides to confront her (the choice). At the restaurant, she challenges the woman (conflict). Unfortunately, the bad guys sent a ringer, and the protagonist is arrested for assault (consequence).
That leads to a new conflict, between her and the system. She has to make a new choice: insist on what seems like a crazy story, and risk being sent to a psychiatric hospital, or play along and accept the blame for something she didn't do.
Whichever choice she makes will have further consequences that lead to more conflict, until there is some kind of final showdown.
BUILDING THE STRONGEST SCENE AND STORY
In each scene we can ask what choices the protagonist has, and which one leads to the most interesting story development. Obviously, the choice has to be consistent with the character you have created, and the character and his or her choices are influenced by the genre as well as the plot.
In action stories, we give the character very few options; in each new development he finds himself faced with some seemingly impossible task he must perform in order to avoid disaster. Think James Bond.
In more sophisticated stories, there are several viable options and which one the protagonist chooses helps us to understand him better and perhaps consider what we would do in his place. In that version, James Bond might stop to consider whether the outcome of the violent tasks being demanded of him are worth the sacrifice of his humanity. Confronted with a particularly vulnerable beautiful woman, he might opt not to sleep with her just to get the information he needs.
HOW THIS HELPS YOU WITH THE MIDDLE OF YOUR STORY
The middle is where many stories weaken. They cease to grow and we feel like the story has been padded. This happens even though there is all kind of conflict and action.
The reason is that the protagonist has stopped making new choices. The story has set up the basic conflict, and that conflict may escalate but if that escalation is just mechanical, the story will stall in terms of its emotional impact.
To see how this works, let's go back to the woman whose identity has been stolen. In the middle (Act II if we're talking about a screenplay), first they take all the money in her bank account, then they make it look like she's been embezzling money at work so she loses her job, then they set up a situation in which she's arrested for assault.
Those are all escalations, but if they are just the result of the actions of the people who are using her, they will not be as powerful as if they are at least in part the result of new choices she makes. For example, maybe she decides, 'If I'm going to be convicted for embezzling money whether or not I've done it, I might as well do it." Shortly after she's been fired, and with her bank account already cleaned out, she takes some of the company's money in order to be able to fight back against the people setting her up. She's made a moral choice that feels like an emotional escalation.
Even more dramatically, if she's going through a divorce and a custody battle, she might decide that her child would be safer with her ex-husband; although it tears her up, she drops her quest for custody. (Hmm, do we think the ex-husband might be in on all this?)
In short, the middle of your story will grow in intensity if the escalation operates on several levels, rather than just the degree of physical threat to your protagonist. Some of the big action movies that lack this try to cover for it with bigger explosions and more impressive effects, but for audiences with an attention span longer than 30 seconds, this ploy can work for only so long.
Whether you use the three C's right from the start, or to help you strengthen your story once you've bashed out a first draft, giving thought to choices, conflicts, and consequences can help you write a more powerful screenplay or novel.