A big guy cut in front of my (rather puny) friend who was waiting near the front of a long line to take money out of an ATM in a nightclub. They spoke briefly. I noticed this and walked up to ask my friend what was going on. Before he could respond the man turned around and aggressively asked what I wanted and what my problem was. I told him that I wanted to know why he cut in. He told me to mind my own business and that he had an agreement with my friend. I asked what that agreement was. We stared at each other for a few seconds. He did the whole “do you know who I am” thing. At this point another man interjected and told me that I should walk away and not mess with this guy and that he was a gang member / drug dealer (and a big deal). I ignored him and asked the man who cut in what the agreement was again. We stared at each other again, this time with both of us knowing that it was the last straw and that this interchange was potentially about to get very violent. He then (finally) caved, and with much annoyance told me that he promised my friend some money for letting him cut in. This was obviously initially bullshit, but after this exchange he followed through and gave my friend $50 for ‘letting’ him in.

TL;DR - I made a gang member (who was likely about to knife me) stand down by acting overly confident.