I've never 'run into' Steve anywhere, but I know people who have. What Ms. Mod says is true, just be nice, cordial, and respectful of any celeb's personal space/time, and usually most will smile back, sya hello, etc. The only famous people I've ever had chance encounters with were Dennis "DJ" Johnson (former Celtics great) and Jackie Mason. In the case of DJ, I just said "Hey DJ!" or something like that and he smiled and said hey back as we passed. While in NYC for a King event, my wife and I crossed paths with Jackie Mason, but I realised it was him only after we passed each other and my brain had a chance to work it out a few seconds later. I said out loud "Wasn't that Jackie Mason?" to my wife, and a passer-by, said yes, it was. We didn't chase after him or anything as he looked like he just wanted to go get his dinner!
Every other celebrity encounter I've had has been at a planned event. Missed a chance to say hi to Salman Rushdie when he and his sone sat behind the two Debs that I knew at the harry, Carrie, and Garp event, again in NYC.