Location shooting was definitely the way to go!

That would be unusual on a real shoot - to not have control over the crowd in the background, I mean. In reality, most location shoots have police sawhorses surrounding the perimeter of the shot, and police and production staff keep the bystanders and passersby on the outside. Sometimes it's a greenscreen - often, a greenscreen is obvious because the background is simply too clear (real shots have foreground/background differences).

I did a few location shoots back when I was still acting, and only once did someone break the perimeter. We were in the middle of shooting the scene and we all heard a ruckus somewhere off near the perimeter, and the next thing we knew, some woman was barreling her way thru the scene. It was funny mainly because we did several takes of the same scene, and that's the take they ended up showing

neat . . . sweet . . . petite

Having done some background work in my past, I can tell you the worst mortal sin an extra can commit is to look straight into the camera. It's a huge no-no, and will get you placed in a dark corner for the rest of the shoot.