How often does it happen to you, that the world you're playing in just seems so...real that you forget about the real thing? Hey, even that monster seems immersed as well! :)
But realism alone won't lead to immersion; most games immerse you into their world simply because theirs is a world that feels logical despite anything/everything game-y about it. What breaks the immersion is a lot of things, like a story decision that goes against what the game tells you to do (like forcing John Marston to die in the end in Red Dead Redemption, when you can wipe out an entire gang in seconds), or an illogical puzzle (like that puzzle in The Longest Journey), or even sudden reminders of game world limits (like those awkward invisible walls in countless games).
As long as the game makes sense to the player, immersion shouldn't be far away. And hours will fly by and you realize your eyes and hands are tired, you're hungry, and you are just human.