Research: Shot/Reverse Shot

Shot reverse shot (or shot/countershot) is a film technique where one
character is shown looking at another character (often off-screen), and then the
other character is shown looking back at the first character. Since the
characters are shown facing in opposite directions, the viewer assumes that they
are looking at each other.
-Wikipedia

This is a very common trick in filmmaking. It's used to create a natural flow in a conversation between two characters, like in this clip from the 1942 classic, Casablanca.

Watching a scene of shot/reverse shot, you'd believe that there are actually two cameras shooting the same scene and then the footage is edited in post-production to cut together both piece of film.
However, this isn't so.
Only one camera is used, filming one half of the dialogue and then the second. For example, Character One would act out their lines, followed by a five second pause instead of Character Two's lines. And then vice versa.
A filmmaker may also choose to film reaction shots to put into the scene as further transitions.

+ The scene may be filmed over a course of days
+ Only requires one camera
- Actors must be able to act convincingly against nothing

It's also important to control the location and be able to recreate lighting, otherwise the continuity is lost.