When I was storyboarding for a recent feature film, I had a mantra, "Don't cut unless there is a good reason to".
Pulling from some ideas from directors such as Steven Spielberg (Jaws) and cinematographers like Emmanuel Lubezki (Children of Men), I was careful to keep eyeballs on the shot by simply not cutting.
Oners prove to be very tasking not only on the camera crew, but the actors, the art department, the effects department and scores of background actors. Pretty much everyone on set feels the pain. But when you finally pull it off, there is really no better feeling of teamwork on a movie set. It's like a carefully choreographed dance with about 100 moving parts, everything has to be meticulously timed to go off perfectly.
In traditional filmmaking, you have your 3 common shots to tell your story.
The WIDE shot establishes the scene. It lets the audience get their bearings on where they are. The MEDIUM goes a little closer and pin points the current related action. The CLOSEUP goes in even closer and tells the audience who they should be listening to or looking at. The art of cinematography "guides" the audience through the Director's story, sometimes gently holding their hand and other times grabbing their hand so hard it hurts then weaving and dodging through the story like a thrill ride.
A Oner, does all of this without cutting. Instead of cutting from the WIDE to the MEDIUM to the CLOSEUP, it uses camera movement or actor movement to form these shots organically. What the audience doesn't know is, it wasn't as organic as they thought. It was carefully choreographed to achieve this goal.
Below is a Oner shot by me, but took about 100 people to pull off. See if you can pick out the different 'common shots' within this one seamless shot. Watch it twice, once as an AUDIENCE member and once as a FILMMAKER. Try and think about the camera and how it moves through the environment and who and what all is BEHIND the camera.