You must force the conversation on someone, for example if you see them first and have no real intentions of killing them yet, hide in their path of travel and pop out from behind and yell at them to "drop everything or ill shoot", Remember there is a possibility of them being in discord and may take awhile to switch to in game chat. Don't do this if they have a gun in their hands. So far this has worked for me 7 out of 9 attempts, the 2 it didn't I had failed to notice that the player I was holding up had teammates nearby. This also can work while they are in a building, just hold the exit point assuming it has one entrance/exit. Many how to videos on youtube as well.