So, a comment from Randy Muholland (of Something Positive fame) on twitter today reminded me of some of my favorite aspects of certain RPG books. The sections that were talking to the GM and dealing with realism. Some of this will be paraphrasing several of those books, but mostly as I've never been able to get these out of my head. Both are from Villains Unlimited by Palladium.
The first example would be the case you get in a lot of super hero RPGs where you have your players - all super powered beings or aliens from other dimensions - look at you, the GM, and wonder why aspects of your game are just so....unrealistic.
The second example would be the case of a player, playing a normal human in a 'gritty' game, putting a gun to his own head when interrogating someone and pulling the trigger to intimidate them. Said player then gets pissed off when the GM rules him dead, because the gun couldn't possibly have cut through all his HP in one shot, mechanically impossible.
What is better is I have run into similar encounters (both of them) in the same game, from the same player. See, realism in games is a weird thing. People have an easier time envisioning things that they know, and so most people's default is reality as we have it here. This is why I ended up in a nerd argument on Jason Marker's blog (man, tons of links today) about what dwarves could and could not do in any fantasy world (find his post about dwarves to get it, I'm not linking it). At the same time, we go to RPGs to escape, to do something fantastical even if it is only for a few hours a week while sitting around a table with some friends.
This leaves the GM in an awkward position. The players are in a world that uses rules vastly different from our own, playing characters that are capable of things normal people are not capable of, and yet they are still expecting some grounding in reality. Some base line foundation of things being like the normal world to help make them feel ok about things. This is why in the world building essays I stress making sure things work together, and that you are consistent. It is ok for something to be different, but it needs to be consistently different and in the same way it was different before. You do this so that the players can see it, experience it, and expect it. You do it because that grounding provides the foundation for other actions and things to stand on, as well as a safety net of expectations when things go off the reservation.
That consistency is good, but you also need a bit more than that. Find out from your players how much realism they want or expect, if you know them you probably have an idea for this but you can also test it by judging reactions to certain things. Obviously some liberties must be taken for genre. Complete realistic repercussions in a Super powered, golden age, super hero RPG doesn't work, while it does when doing a nitty gritty balls to the walls real world story. Somewhere in between your genre, your game, you, and your players though is the right amount of realism. Some of it is just common sense (objects fall when dropped unless otherwise held up), other things do not need to be. Hell, some things in the real world just don't make sense in the first place, so why should everything in yours? (I point to armadillos being able to swim).
So find your balance point for your game, and don't be afraid to defend your ruling by pointing out the absurdity of expecting realism in all things when the PCs themselves are not realistic. Or as Randy responded to the last few claims of things not being realistic in his comic, "Pudding Cat!"