Research- Rule Of Thirds.

In film, the rule of thirds is a concept in which the frame is divided up into nine sections, these sections are imaginary. This creates reference points which act as guides for framing a shot.

In most shots of people, the main line of interest is going straight through the eyes. The eyes are normally positioned 1/3 down the frame. However this really depends of the type of shot and if its actually possible to place the characters eyes like this.
The intersection of the two lines is sometimes called the 'power point' or 'crash point'. Points of interest in the shot dont actually have to touch these lines to take advantage of the rule of thirds.