Capturing people on paper is fascinating to me as well Ernest. And the more you do it, the quicker you become at getting a gesture or position down correctly. My suggestion is for you to avoid detail and concentrate on the position of the body and limbs. Use simple lines to get those in position and if there's time after that, then go for some detail of clothing, facial features, etc.
I draw a lot in coffee shops, airports, etc. and find that you get a minute literally, if that, to draw a person's position, you need to work around movement, shifts in position etc. I find people tend to repeat the same movement or position over and over so if they are sitting in a coffee shop, you can get the general position then wait a fill in more over time as they will resume that position again several times.
And YES! go to a life class. Sign up right now. Yes, right now! Go on...you know where the phone is...