What you see vs what you know

I've been posting a lot of quick sketches and gesture drawings. I've been going out on Saturdays if anyone is in the LA area and wants to join, hit me up on facebook. I usually post where we go. anyways, there is a difference between copying what you see, like in a long pose, and modifying a memorized version of things in a quicksketch. the best way to explain it would be in facial features. I have a set of memorized eyes, noses, ears, etc. I don't copy the person in front of me, but say they have a large or small nose, I draw my memorized nose small or big. I also try to push any ideas I might see further. The reason I do this is, sometimes the person is riding by on a bike, or moving and you want to catch a quick pose, so it's good to not be looking at them for the stuff, but just the pose. Then you can glance once, get the gesture and then just sit and draw your drawing