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To position actors correctly in the frame you need to make sure their eyes are in the right place. Get this right, and the audience will always know where the characters are located in the scene, and when they are looking at each other.
This video tutorial will show you how to split the frame into thirds, in your imagination, so the eyes are always aligned the right place. The technique gives you great freedom to change the angle and position of the camera, so long as they eyes are always framed correctly.
You'll also find out how to keep the camera on one side of the actors, by drawing an imaginary line between them. This ensures continuity when cutting between different angles on your actors.