Look, you might not know it, but your life is boring. Tedious, tiresome, dull, monotonous, uninteresting, trite. You might not realize it, as you are living it, but if anyone would have to watch your life for more than a minute at a random moment of your life, what would they see? You sleep, work, eat, shower, use the bathroom, consume some entertainment often passively, stare at your computer’s screen, whatever. Your life is very, very boring. Its probable that even your sexlife would be boring from the perspective of someone simply watching it, unless they have very voyeristic tendencies.
So, this is why we don’t write about, or play normal people. They might be “normal” capabilitywise, but they definitely aren’t normal. We can use normal people as basis, but we need to exaggerate their qualities. Instead of being a bit down once in a while, your depressed character needs to be suicidal, instead of drinking in secret during the day, your alcoholic character needs to be obscenely drunk all the time, instead of a guy likes to put down numbers and make calculations, your scientist character needs to speak in something closely resembling technobabble.
But there are limits. You don’t need to remind everyone all the time about this. After initial setup, a little goes a long way. If you keep bringing up the peculiarities of your character, he or she will slide into being comic relief. Who wants that for their character? Maybe for a oneshot, but I wouldn’t like to do that all the time, although I will often do stuff for comedic effect with my characters. I just try not to overdo it.
Also, normal people have responsibilities. They have jobs, families, mortgages, church groups, whatever. Normal people don’t really lend themselves for taking time off to go adventuring. How do you explain that to your wife? Are you going to be able to handle the cost of the medical expenses? What happens to your family if your adventuring results in you going to jail? Basically, you just don’t play them.
Of course, certain genres play with the idea of putting normal people into difficult situations. This is something for oneshots, because you don’t want that kind of baggage for campaigns. (Unless it sits well for reason.)
Obviously, in the end, no one is really “normal”, so we can just drop the facade anyway, and let people be outrageous with their characters… as long as they don’t emphasize it too much.