So the context is I'm in Rome near the Pantheon, a simply brilliant area with a cool vibe. I notice a professional photo shoot taking pace, model, lighting, various outfits ready in the wings. I just cannot help myself as the image I can visualise is just too good. I wait at a respectful distance and when the model looks me in the eye, boom I get the photo I wanted. It's all in the eyes, that brief glimpse is enough for me to make a photo. So be respectful, often I have walked on not wishing to disturb, but if you can wait until you get the eyes looking at you. That works for almost every portrait photo I can think of, after all the eyes are the window into the soul.