When a really cute little girl asks you to play a song during your "there are too many trains right now and I'm tired" break, you get up and you play. And when she asks you to play Nicki Minaj, well you figure out the chorus to "Super Bass" on the fly. Well, at least I do. And even though you are most decidedly not a fan of pop music, when she asks you to, you even sing "Baby" by Justin Bieber (of which you ashamedly know almost the entire 1st verse and chorus).
This is an important lesson for me and any aspiring buskers out there: if the song is popular, even if you don't exactly like it or think it suits your style, it's helpful to know at least the 1st verse and chorus. Hey, "Baby" even got me a dollar. ;)
(And if you're a black female, especially if you have locks, just go ahead and learn some Tracy Chapman. I suggest "Fast Car".)
Now, on to duets. Sometimes you'll get really cool people who ask to join you. This handsome fellow pictured below was one of those types. We did a wonderfully harmonized rendition of "Landslide" (1st verse and chorus) before his train came and I had to finish the song solo, sans harmony. I can't remember his name, but it was a nice one.
Busking is really great fun because you get to meet so many different people and you stretch and grow as an artist and musician every day. Basically, if you can work up the nerve to prop yourself up in front of dozens of strangers in a non-venue setting, then I highly recommend it.