I try to kind of simulate that by being unarmed if I dont have the gun on my hands,and by not being that guy that walks around flagging everyone with his weapon.When something happens,I have to press the right key or open the pipboy and pick the gun,and then proceed to killing.Of course I still kill them,but every once in a while a NPC manages do get a good shot off before I get my gun out.If you already have it on you,it feels like cheating: you just press Vats and thats it!Mine is definitively not a solution,but its better then nothing,I guess.