a path which your subject is traveling along or an imaginary line between two characters who are interacting

Is that portion of the common tangent to the base circles along which contact between the mating involute teeth occurs.

The line of action of a force is the infinite line defined by extending along the direction of the force from the point where the force acts.

An imaginary line used to help stage camera positions for shooting action. Typically 'drawn' along the line of sight between two characters in a scene, or following the movement of characters, cars etc. In the continuity system all shots of the action will be taken from one side only of the line to maintain consistent screen orientation and direction of movement.

A line of action is merely the direction of a force. Because forces are vector quantities, the direction of the force vector (and equally, the acceleration vector) will be equal to the line of action.