One way to make a candid portrait is to sneak the photograph. Sometimes it works, but for me it's usually a failure. A better way for me to make a portrait is to make friends first. The trick is to avoid being too direct and thus rude.

My method for making friends is to spend some time in the same place. I keep my camera out of sight, or at least out of use. After a while I become part of the scene, rather than an intrusive stranger. When that happens - and it's a moment whose arrival I have to intuit, it might be less than a minute or several minutes - I can ask to make a photo, and after a little while I don't even have to ask.

Sometimes I'll bring along a photo album - pictures of my family and friends, and perhaps people I've met on my photography trips. Generally, if I'm able to show someone my photographs, that person wants to be included in my album, too.