Where does the expression come from? “Actor A is starring in The Bestest Film Ever, opposite Actor B”, that kind of thing.
I think I have seen this mostly in “romance” type movies/plays where there is one male lead and one female lead who are “opposite” each other.
I don’t recall seeing this used with two male or two female actors in a film.
I may be wrong, and YMMV.
According to the OED (bolding mine):
First cite in this context is from 1936:
“Sense D” in the original definition is:
The first cite here is 1931. I’d assume it derives from “facing” in the definition.