Learn English – “Opposite to” Vs “Contrary to”

Not in the way you are using it.

"Small is the opposite of big" is saying that the two words mean opposing things. This is okay, because "of" indicates a difference in essence.

"Small is opposite to big", however, indicates an opposition in physical position. It suggests that "X" is in the opposite position compared to "Y". The "to" in this phrase indicates a physical opposition, where the words "small" and "big" do not have physical entities to represent them, and it would only make sense if you were putting the words physically in a list, and saying they were opposite to each other on that list.

Opposite means “on the other side of”.

Someone who is cast opposite another actor is going to play many scenes on the opposite side of the action or dialogue, relative to that actor. But that does not mean they have to be antagonists (the correct word for the character that acts against the other actor's character).

In the case of casting, opposite does often refer to an antagonist, mainly because mainly story lines have a protagonist and an antagonist as the main characters. If you have two protagonists in the lead, you can still use opposite if the acting physically seems to place actors opposite from one-another.

I can think of an example where one character is another character's psychiatrist, and doctor and patient are surely not antagonists in the story. Since their cooperation happens from both sides of the doctor-patient relationship we can still imagine them as being cast opposite each other.

In case two characters fully cooperate in the story, you will usually say that actor X was cast alongside actor Y.