A few years ago I started to document people's faces in photographs. I was interested in the early daguerreotype portraits from 180 years ago. The intense, monotone pictures of people staring back at you from a distant past. I was drawn to the simple eyes-to-camera poses. Daguerreotype 'photography' required people to sit still for long periods of time, which often led to stilted, very static portraits. These days I don't think people ever spend more than a few seconds 'posing' for a picture.
I'll often sit somewhere busy and simply watch the faces moving past until I see someone that captures my interest.
I think you have between five and ten seconds to sell yourself to a subject. You need to explain who you are and what you're doing while also demonstrating that you can be trusted; that you're safe.
Telling people I'm shooting portraits for a project called Faces of Brighton has been a great help. People intuitively understand what I'm doing.