I've been a people-watcher my entire life. As such, I've learned by watching and by research, both reading about and conversing with people, on what leads up to contact with another person, be it friendly or otherwise. There are many tell-tale signs that lead up to this event and once you learn how to spot it, you'll be more ready for it when it happens to you or someone you know.
First, the person will be looking around. This person, friendly or not, is observant. He's either looking for someone he knows or looking for someone that has what he wants. It is up to us to be observant as well and not glaring at our phones or turning with our backs fully toward everyone.
The next thing we will see is their selection. They will either lock eyes with the person or at least watch the person closely to determine their next action. If they have friendly intentions they will likely walk straight to the person to get their attention. This is where the friendly interaction will cease it's similarities. If it's other than friendly, they'll look around to make sure they won't be seen by others.
Next, people with nefarious intentions will follow their target to attempt to isolate them from others. The fewer people around them, the less likely they are to be caught. The target will likely still be distracted while this is happening and not realize they are walking into the trap of the attacker.
When the target is finally isolated, the attacker will approach them and speak to them to distract and put them at ease for a moment, typically by asking a favor of some sort, such as directions, borrowing a phone, or even a stick of gum. It is at this point the attacker will attack, either physically or through threats and demands of property.
If you find yourself in this situation, remember that no property you possess is worth your life. If you see someone in this situation, be smart. You don't want to cause any more harm to yourself or the victim than is necessary. We always need to offer help to those in need but jumping in to be the hero can result in a startle response by the attacker and do more harm than good.
If the demand is property, money, watch, jewelry, etc, let the transaction take place and call your local emergency number. If the demand is personal, asking of certain "favors", assault, or attempting to kidnap the person, now is the time for action, whether it is you or someone you saw. Again, be smart. A startle response by the attacker can result in an easy trigger pull, stab, slice, or any number of other injuries to yourself or others. Any encounter you or the victim can walk away from is another day for each of you to live.
Alert security or employees or the possible target whenever you see suspicious activity and do your best to keep yourself and those around you safe. Think before you act and avoid being distracted in public.
Be alert, be aware, stay safe