Fallout

I think that depends on how big their role is in something. If an actor is just reading a monologue or a few lines as part of narration, it's probably not worth the time. For a major role which will interact with the world and lore, I'd agree that some background work would be useful to understand who the character is and what their place in the world is. Though, at some level, people are people regardless of their specific situations. If you understand (or build) their motivations in a reasonable way, then their actions and reactions can be understood by them reacting to the world around them in expected ways. Sure, some larger than life people might react in very specific ways, and a different upbringing or training can shape how one reacts, but it's still going to be about understanding that character's motivations.