I've included in this post some examples of sketching bodies in real life/real time. You can see they're not that impressive. Subjects do what they do, so sometimes I've only got a minute, if I'm lucky, to get down a gesture or a memory of a gesture. I probably spent 30 seconds to 2 minutes on each of the figures in the first five drawings below. Except for the sleeping guy, everyone was pretty much on the move. It's a really good exercise in trying to capture motion on instinct. Sometimes the result is something simple and fresh that I wouldn't have gotten if left to my own muddling ways. You can see the difference between these gestural drawings and the last two drawings, which I did from photos. I spent a little more time on the expressions and got "creative" with the environment and social interactions because the information from the photos are there for me to play with in my own time (about 15 minutes per sketch).
I recommend drawing both from real life and from photos and try to blend the two elements together!
**edited - sleeping guy sketch is totally there now. I can't believe I forgot him.