Shot/ reverse shot is called 構図 KOUZU/ 逆構図 GYAKU-KOUZU in Japanese. 構図 KOUZU means composition. 逆構図 GYAKU-KOUZU means reversed composition.  I discussed in the previous entry that If you need a precise reverse shot (GYAKU-KOUZU) of a shot (KOUZU), all you have to do is:

1) Use the same focal length lens. (Magnification)

2) Maintain the same distance between camera and the object. (Distance)

3) Keep the same angle between imaginary line and camera axis. (Angle)

According to this theory, we end up having two results (see the diagram above). One is Jude Law in the reversed composition of Cameron Diaz and the other is he in the same composition as she. Two in the same composition may appear to look at the same direction or at least disorient viewers. Now, let me add one more to the other 3rules. 

4) Don't cross the 180-degree line. 

In essence, if two characters switch relative positions on the screen from shot to shot, it causes disorientation and confusion. While during a shot, it doesn't. 

Two ways to switch their relative positions during a shot

1) Actor's movement

The Champ (Franco Zeffirelli, 1979)

Billy (Jon Voight) tries to convince Jackie(Jack Warden) of his come-back at the restaurant.

C#1,3 Jackie is facing left on the screen right. (Shot)

C#2,4 Billy is facing right on the screen left. (Reverse shot)

C#5 Billy moves from left to right. Jackie now faces right on the screen left. (Shot in which they switch relative position)

C#6,8,10,12 Billy is facing left on the screen right. (Reverse shot)

C#7,9,11,13 Jackie is facing right on the screen left. (Shot)

2) Camera movement

The Holiday (Nancy Meyers, 2006)

Heat (Michael Mann, 1995)

Why do they go that far to switch their relative positions? 

1) For Background

In The Champ, first-half shot/ reverse shot shows the bustle of the restaurant as their background. After Billy moves in C#5, his background changes into the wall. The wall lacking depth isolates and emphasizes Billy. This change comes just when Billy makes a counter-charge against Jackie's opposition.

2) For Lighting

As you can see in Promised Land , camera crosses the line two times during each shot by moving itself. Ostensible conversation starts with front lighting. When the politician subtly demands a bribe from Butler (Matt Damon), camera goes to the opposite 180 degree arc in which they are silhouetted against a large window. After their negotiation is settled, camera goes back to the original 180 degree arc with front lighting.

3) To show Left side face or Right side face

In The Holiday, Amanda (Cameron Diaz) on the screen left interviews Graham (Jude Law) on the screen right at first. When they change their roles, camera goes to the opposite 180 degree arc showing their different side of face mainly. The interviewer is always on the screen left. The interviewee is always on the screen right. 

4) To show Inside or Outside

In Heat, during Neil (Robert De Niro) gives the cold shoulder to Eady (Amy Brenneman) out of caution, shot/ reverse shot is taken as over-the back of-shoulder shot. After letting his guard down, camera goes to the opposite 180 degree arc giving us over-the front of-shoulder shots. 

As stated above, there exist the right timing and reason to have to cross the 180-degree line.