Learn English – When something unexpected happens in a film, can it be called a “reversal”

Tardy

Occurring, arriving, acting, or done after the scheduled, expected, or usual time; late.

Microsoft's tardy response to iPhone – After its debacle with the Kin, Microsoft has gotten its smartphone act together with the announcement of its first Windows Mobile 7 phones (10/11/2010)

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.