Portraits: People at work or leisure

Here's a young reader at Borders, in Chicago (M3, Elmarit 135 on Fuji Superia ISO 400):

And here is an old calligrapher in the Art Institute of Chicago (M6TTL, Summicron 90, T-Max ISO 400):

And last, a young vegetable vendor at the Tibás weekly open air market in San José, Costa Rica.  She was kinda shy (M6TTL, Summicron 90, Kodachrome 200):

I like these shots because they're kind of candid portraits, pretty much like the candy vendor in San Juan, PR (see below).  Now, not all were taken in the street but all were done with one purpose in mind: to document people in their daily life.  Granted, the calligrapher probably doesn't do what I photographed him doing all the time, but if he is one who is always called to demonstrate the beauty of Chinese characters, we can safely assume he is doing something familiar and relatively ordinary to him. 

What goes into the perfect portrait?  Let's get some ideas...