De Niro was the easiest for me to i.d.
I grabbed some popcorn from an official cart at City Hall Square Park (was that three years ago?), held the popcorn container in front of me, looked calm, and wandered into some kind of media event. I was a bit startled when I got in.
New York's kinda cool that way. I literally walked into Julie Andrews in Gramercy Park at a book event the next year. We bumped into each other - I was rushing to one reading, she was going to her own - she was awfully nice as we had to work our way around each other.
I seem to always spot 'someone' at the Union Square Farmers Market (or going into the Whole Foods on the south side). It's like gossip mag central right there.