A scene is a unit of conflict.
Every scene is a story. It has a protagonist and an antagonist in conflict as they struggle for their goals. These goals may be very small, their struggles very slight, but scenes need those struggles and goals for the same reason that novels and movies need struggles and goals: to keep the story moving.
You’ve probably noticed a pattern here: linear structure is composed of narrative units, each unit having a protagonist and an antagonist in conflict as they try to reach their goals. Story plots, subplots, acts, and scenes all have that structure. Those conflicts are broken down into units in each of these forms: Plots can be analyzed by breaking them into acts; acts can be analyzed by breaking them into scenes, and scenes can be analyzed by breaking them into smaller units, sometimes called beats. (Next on Writing/Romance: Beats).
Although the major structure–novel, novella, short story–will have only one main protagonist and one main antagonist, the scenes within those larger structures can have any character as a protagonist, who is almost always the point-of-view character in that scene.
The key to scenes is the same as the key to acts and to the entire novel: Every unit of conflict should change the main characters and move the story forward by escalating the plot and setting up expectation for the next scene, even if those changes and movements are very small.
If your scene seems dead, try running a conflict box on it. The vast majority of the time, the problem with a boring scene is weak or no conflict.
For other posts in this unit, see The Structure Unit main post.