Final answer:
The small building where actors change masks is called a Mask booth in a proscenium theatre.
Step-by-step explanation:
The small building that was likely used for actors to change masks is called a Mask booth. In a proscenium theatre, the Mask booth is typically located at the back of the acting platform to provide easy access for actors to change their masks during performances. It is an essential technical space that ensures smooth transitions and allows actors to portray different characters effectively.