I'm a serving police officer in the Hampshire. We are trained to deal with suspects as calmly as possible, without creating more work for ourselves through antagonising the subject (regardless of what crime they are suspected of at this time). I always address people respectful as sir or if more appropriate mate, pal. When I need to call people over to to talk to them, it really depends on the situation on what I say. More generally I would say something such as "Hello sir, I need you for a minute, just routine mate" or during an arrest its imperative to keep the suspect on side. "Put your hands behind your back for me mate" and when they comply its good to commend them somehow as they can be very unpredictable with us, especially as a lone officer from time to time. "Good chap" or I often get personal with them asking them how they are or why they have themselves here today. To be clear I am a UK police officer and I do not carry firearms or TASER meaning I am not always in the position to try tactics used by American counterparts. Just my input, hope it helps.