Gary Oldman AS Winston Churchill vs. Gary Oldman IS Winston Churchill

Gary Oldman is not Winston Churchill. The latter person died in 1965. Oldman is an actor in a film. However, when advertising a film, you might use an emphasised "is" to suggest that an actor's portrayal is so convincing that a viewer might almost forget that it is an act, e.g. Richard Burton IS Mark Antony!

You can say
Gary Oldman is Winston Churchill in the film "Darkest Hour".
or
The film "Darkest Hour" stars Gary Oldman as Winston Churchill.

Yeah, once it has been established that the context is a film, a play, or whatever, people will often say "is," with it being clear that it's not to be taken in a literal sense.

In this same context, "as" just means "plays the role of."

The meaning conveyed is the same, either way.