lighting - How to light spontaneous portraits?

I currently am in Singapore for an exchange year. The architecture here is impressive, the lights at night are wonderful, but what I find to be the most interesting are the people. You can find people from all races, sizes and ages here.

I would love to just ask some of them on the street, in the bus, in the MRT stations or in the malls if I could take a photo of them. However, I guess, a little preparation can't hurt as I'm not too experienced with portrait photography - especially with the lighting.

Can you give me some tips on how to light spontaneous portraits? This means I can't put up a studio environment and don't have much time to adjust the lights. Furthermore, I'll have to deal with an array of different lighting situations (outside, sunny, cloudy, at night, in a mall, etc). Last but not least, I don't want to scare people away with huge amounts of equipment!
Should I buy a flash, and if yes, which one?

Of course other tips on both interaction with people and the technical side are welcome, too.

PS: My equipment is a Canon EOS 450D + EF-s 60mm / f2.8

PPS: Thanks to everyone for your tips! Here is one of my first shots:

Answer

You light spontaneous portraits with available light. Sure, flashes, strobes, kinoflos, etc are great tools and really expand your visual vocabulary, but at the end of the day, nothing is as authentic and versatile as available light.

Start observing light. Find a cool light source - be it a back lit outdoor ad, a store display, etc. Look at how it lights people. Go with a friend, ask them to move around the light as you watch them. See how the light breaks, how it changes their features. Have a couple go-to moves - a direct ceiling light from above, a shot incorporating street lights, maybe a shot in the shade with a blue sky in the background, the specifics don't matter. Just don't stop experimenting, there's so much awesome mixed light in most city environments, it's a shame to overpower it with strobes just because that's the easy out.

If you absolutely must buy gear, get a Speedlite - a 580, 430, doesn't matter - and a cable. Being able to remove the light from the camera and movie it around your model is absolutely crucial. Learn to control ambient via shutter speed and flash exposure via aperture, there's a lot of great info on doing that on strobist.

And remember, there really are no correct answers, photography - especially non commercial photography - is all about expressing your vision. Have fun with it!