Aren’t the backs of people interesting, watching a woman looking at a steamroller, or a couple walking along the canal towpath, hand in hand.
When you try and sketch figures quickly you have to remember their positions because they are sometimes moving. It’s easier when they stand still for a while, but that doesn’t mean they are motionless. Heads move from side to side, or the weight moves depending on which leg they are standing on. Then the arms move, hold, wave about. A few quick lines are not always enough, but I try.. Two or three minutes tops for these, then colour added later.