
Frank T. answered 04/20/19
Ph.D. in Film & TV Studies (UCLA) with 35 years of Teaching/Publishing
Sometimes two cameras are used during a dialogue scene but that can get complicated because of the differing lighting needs of the two angles (the lights for one shot may cast a shadow on the other shot).
So, generally, all the shots on character A are lit, rehearsed, and filmed from one angle -- at the same time, and then all the shots of character B are shot. This saves a lot of time, because you don't have to switch over after every few words; just film continuously on each actor.
Usually, a "master shot" showing both people is shot first -- and those light can generally be used for the close-ups. Over-the-shoulder angles (two-shots) can also be shot before setting up for the close-ups.