Sweeps

I have a question regarding sweeps because its hard to grab someone in the gym and ask them to demonstrate them due to their unpleasant nature for me. If someone is walking away from you and you want to sweep them under their legs while they are walking, would you try to make them immobile first like grab their shoulder with your arms while you cut their legs off by kicking behind their knee joints with side of your foot? Or would you just turn parallel to them as you walk and they don't notice you and sweep at the same time as you grab the shoulder for balance?

Also, if their legs are spread far apart do you sweep just one of their leg knee joints or do you go for the ankle? Because when their legs are close you sweep both I know that but Ive noticed some people on the street or stores stand with far stances.