How much realism to add to a game? It depends on the sort of game that you're making, actually. Mysteries, Crime Dramas, and such, you actually CAN get away with adding as much about solving crimes, forensic or even law if your character is a lawyer, or even in law enforcement. Of course, with those characters you'll have to have them act like they are in those fields. Horror games, no one acts like they do in real life. *Crash* Teen girl: Billy, I heard something... Billy: (game) I better go check it out. *Real life, Billy* Sarah, we better call the police and have them check it out. Or heck, they might go to the neighbors as the police officers arrest the person trying to break in.
Fantasy games are really open, because you have magic, summonings, so you can't have realism... Well, yes and no. I would say that as long as you create maps that are rich, full of life and is believable, you can add the most important part of realism to your game. Your characters reactions to things, they have to believe it, so that your player can believe it. Science fiction, on the other hand, feel free to add any science information within reason as it will add to the realism to the story.
But the most important part of realism is going to be your characters, their lives, their reactions and making it seem like they really live where they do. For example, small towns where most of your heroes are likely to come from. Everyone knows everyone. Stories and gossip are spread pretty fast and at times, people get into everyone else's business. You are likely to have farmers in your past, if you lived in the area long enough. Teenagers have their 'hang out' spots, while teenagers in the past, became apprentices in a field. (o_o) In the city where I live, teenagers used to drive down Main Street, while today, they mostly hang out at Walmart. What can I say? If you're not a wilderness buff, there really isn't much to do around here. It's the little things in life that you can add to make it feel real.
Status, class, where you grew up, family