From how I see it, he is an actor. He is expected to get a script and act accordingly. Depends on the director and others how open they can be about it and how much freedom the actors get. But I don’t talk from experience.

Now as a Witcher fan, I understand Cavill completely. He cared for source materials and got writers and showrunner who didn’t. Some writers allegedly even actively didn’t like the source material. So of course he would fight it. I can imagine ways that are good, and I can imagine ways that may be stepping out of line from writers perspective.