Ah, now this I can understand, from both sides.
Upstaging doesn't probably happen much in film, because there is no audience and also the moves have to be very carefully blocked out for the camera. It can still happen, but maybe not so overtly.
First of all,
Upstage is the section of the stage furthest from the audience. This back part of the stage used to be set higher than the front, like a ramp down to the audience, thus the word "
up-stage"). If someone upstages you, it literally means that they step toward the back of the stage and face the audience....even if they do this ever so slightly, it forces the person or people they are performing with to turn their back to the audience in order to look at their fellow actor, making the stepper-back the focal point of the picture. The term has come to stand for all forms of taking the spotlight from another person, such as doing an intricate bit of business while someone else is giving a big speech, mugging, or speaking in an odd manner to get attention.
Here, Bobby Driscoll is upstage or behind all the other actors - he is the focus of the scene.
Here, there are extras further upstage of (or behind) Boyer and Garbo, but the couple is still the focus of the scene because of the skewed perspective, and the ring of white around the couple. Also all eyes are on the couple.
There are things you can do to an upstager to draw the focus back from them to yourself:
Here, Angela Lansbury is upstaging Ingrid Bergman (on purpose though for the camera - it's what her character would do). But Ingrid has pulled some of the focus back by facing the audience herself, not looking at the couple behind her.
I suspect that Boyer had bits of business he used to make a more rounded characterization, and if Colbert was at all nervous about performing, she might have thought he was hamming it up. I also think that Colbert's demands marked an insecurity, but Boyer would not have liked it, and so she set the tone for their problems with one another. Maybe she made the other theatre personnel's lives so miserable during rehearsals that he felt upstaging was justified. To force the budget through the roof is not a very good way to get popular with your fellow theatre people.
