People going upstairs tend to scrunch down just a teensy bit before each step upward as they prepare their muscles for the exertion of the next step. This is almost reversed on the way down. They tend to stretch down just a smidgen to the next step. You don't want to over exagerate this, or they'll look like they're sneaking, but you don't want to underexagerate it, or they'll look stiff. :o) There is a tiny little bounce at the impact of each step going down, the amount depends on the poise of your character. Oh, also the chest will tend to shift ever, ever so slightly from front to back as it compensates the exchange of weight for it and the head. You have to think about where the weight is focused in relation to the feet supporting it at a given time and which way the chest needs to be to balance it all right. Now skipping and running up and down stairs have whole different dynamics and I won't go into that here.