MEDIUM SHOT
The medium shot is the most common shot used in movies. Every shot that isn't a long shot or close up is a medium shot. Medium shots can be shot with any lens, but generally a medium lens is a lens that has the least amount of distortion, which is the same as to say that it photographs people and scenery in the way you normally see them in true perspective. (In 35mm photography the common medium lens is the 50mm and in 16mm photography the 25mm lens.)
John Ford is famous for relying heavily on medium shots in shooting his movies; but, that should be considered an exceptional style, a sort of stream of consciousness type of thing, not everyone can pull it off. Furthermore, the reason given for this style was so that editors couldn't fool around with the structure of his movies. He claimed to never go into the editing room or see his finished movies, so he wanted to make sure that there was only one way an editor could put his movies together.
That does make sense. There is rarely more than one way you can cut a movie that is made up of one camera angle.
I once was given an assignment to edit a movie that was all shot in master shots. I was crawling on the walls trying to make the cuts work. It was a miracle that I was able to put the film together. There were no cutaways or anything! And the film was picked up by a distributor and sold!
The medium shot should generally contain all the action of the scene and it should be well matched with the flow of the long shot, so that the editor can cut smoothly or effectively at practically any point between them. This is the case especially if they are both masters of the same scene.
If the medium shot is a continuation or a further development of the story contents captured by the long shot, then the medium shot should be matched to some characteristic of the long shot with which it is supposed to be
intercut.
Medium shots can be waist-high "singles" (covering one actor,) group shots, two-shots or over-the-shoulders, or they can be shots of any subject matter, as long as the framing looks "normal" to the human eye.
Over-the-shoulder shots are the most effective medium shots, because they lend themselves best to be cut to close-ups, since the perspective of the photographed face doesn't change very much in the cutting, thus allowing for a smooth transition.
Also, the over-the-shoulder has more immediacy when used at extreme angle where the actor who's dialogue is filmed over the shoulder of another actor is practically looking straight into the camera (communicating most intimately with the audience!)
Two-shots in profile are not as effective as over-the-shoulder shots; how-ever, they are important in dramatic confrontations!
The other two-shot is two actors sitting in some side by side manner. Take the FORREST GUMP scenes of Forrest with his box of chocolates, sitting on the bench with other characters, in two-shots. Both actors are in full face view and in very simple and yet effective exposition.
A more effective two-shot is of Burt Lancaster and Deborah Kerr kissing in the surf in the classic FROM HERE TO ETERNITY.
As important as medium shots are as building blocks of a movie, avoid over-using them. That sort of thing amounts to sloppy film making. It's a lazy way to approach a story. Remember, your main goal isn't to prevent the editor from doing his thing, because you might be the editor!