Line and Shape in cinematography
Invisible lines in shot composition – the rule of thirds
In the MZed course “Fundamentals of Directing”, independent screenwriter and director Kyle Wilamowski explains this phenomenon quite simply. Take any desired frame, draw two vertical and two horizontal lines across the screen, dividing it into identically sized thirds, and look at where they intersect. These are the focal points that immediately capture the viewer’s attention.

Leading lines to guide the viewer’s attention
Leading lines can be horizontal, vertical, or diagonal. As the name “leading” suggests, they smoothly direct our glance toward something the director wants us to focus on. One striking example is Stanley Kubrick’s “The Shining” and the scenes in the hallway.

Even when little Danny rides his bike through the empty rooms, our eyes keep wandering to the middle of the frame, to the point in the distance. That’s how the classic central perspective works, creating diagonal lines on the sides of the picture. As a result, we unconsciously anticipate that something will happen there. When the twins appear, they are positioned precisely at this spot within the frame, and they become the main point of interest.
Trapping your characters inside the lines and forms
That’s another reason why you would place a divider inside your frame. Lines can comment on a story situation and intensify a desired emotion. Say, if it’s a character who is feeling trapped – lines are at your service. Hulu’s series “The Handmaid’s Tale” exemplifies the use of this technique. See for yourself:

On one hand, the creators of the show really love to work with closeups so the audience develops an emotional connection to the women enslaved in Gilead. On the other hand, when they do show them in wider shots, they often place the characters trapped inside line elements, like in the examples above. What’s interesting here is that it doesn’t even have to be rectangular to create this anxious feeling. The cage formed out of their arms, closing in on the protagonist, or the diagonal lines of bed curtains framing the wife (who is also a prisoner in this situation, at that moment), have the same effect.