Voice actors get the lines and direction for all scenes they are part of, as well as any barks they will be doing (e.g. attacking noises, taking damage, attack call-outs, etc.). What they say during interviews is mostly their speculation based on what parts they've seen of the scripts. The voice actors usually perform opposite a stand-in to speak the other lines. The more scenes they get, the more they can piece together what the game is about. This is primarily because VAs have historically been a big source of leaks and spoilers in the past (which is why one of the publisher demands during the last voice actor strike was harsher enforcement of NDAs). Several better-known voice actors do voice work for many, many games so they will often just do it with minimal preparation. since they've got so many to work through.
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