Well yes, if the game was trying to aim for full realism sure. Just like an NPC ought to notice a huge rock "sneaking" up on him/her. Thats simply the style of game this is. Not only by estetic choise, but its also not yet possible to have actually intelligent NPCs in games.
There will always be areas where you are simply going to haveto suspend your disbelief and go with the internal logic of the game. In this game, if an NPC is in a dialogue, that NPC cant interact or do stuff untill the dialogue is over. Even if things were coded the other way around you would be able to create other weird situations where you can interrupt iportant story dialogue by doing stuff in the background with other characters.
