I painted this on site during this year's Sonoma Plein Air Festival. I had my easel set up across the street from this cafe, and painted this in about an hour and a half.
As I was working, a few people stopped by to watch and they all asked how am I painting moving people? If it's a non-artist asking, there's usually some joking around; "Gotta be pretty quick, huh?" or "did you pay those people to sit still?" Every time I include people in my plein air street scenes, I hear these comments at least once during the painting process. I groan inside but I don't want to be rude, so I just laugh like it was the first time I've heard anyone make that joke.
But sometimes an artist will stop by and ask how to do it. And I explain to him that each figure is a composite of a bunch instantaneous impressions. The people sitting and drinking coffee at the tables don't move a whole lot (relatively speaking) so they're not too bad, but the waiter coming and going is a little tricky.
When you think about it, all painting is done from memory. You look up, you take in the information, commit to memory, and you look away. You can't see both the subject and your palette / panel at the same time, after all.
I try to focus on gesture, above all. Sure, you need information on light and shadow, colors and values, not to mention shapes and scales, but the gesture is the one thing that's fleeting and it's the one thing that communicates movement, a sense of life.
Because the waiter's job is repetitive, he'll strike a similar pose again and again, allowing me to study his posture each time, and make adjustments to my efforts.When I'm focusing on gesture, I look at the figure and try to memorize the general shape and flow of the main parts of the body. I try to draw to communicate what the figure is doing, not what he looks like. And this mind set is key. What is the back doing? What is the arm doing? Which foot is supporting the weight?
Also, when I'm focusing on gesture, I'm not thinking about color. When I'm thinking about color, I'm not thinking about gesture. These things can be done on separate glances. If I had to think about gesture and color at the same time (and value and edges and opacity and viscosity and texture and...) I'd just get confused.
But when all is said and done, the only way to get good results is... you guessed it, practice. Practice your short-pose gesture drawing constantly! Go to open sessions and do short pose (1 to 20 minutes) drawings at least once a week. On other days, you can practice in your sketchbook as you sit in a coffee shop or in your car waiting to pick up your kids. Or whatever.
Or you can take a photo. But I'm sure I don't have to tell you, it's an entirely different experience. There is this immediacy in working from direct observation that is immensely enjoyable and satisfying.
Even if the painting doesn't turn out, I get a kick out of trying to paint moving subjects.