Lets say I'm shooting a character which is surrounded by a white background (or green screen), but the background is not wide enough to fill the whole image. What can I do to enlarge the edges of the background?

Look at this video. This guy was shooting in front of the white background which was smaller and moreover not so clean and perfect as in the final image. How can I do this? And how did he move the character around?

Lighting Infinite White on Vimeo

I would just use Pan/Crop to crop off the left and/or right edges so that the next track down will show through. Put white on the next track down.

Track Motion can be used to move the character to different locations on the screen.

I have a similar setup in my studio but I use interchangeable backcloths (black, white or green) rather than painting a solid surface.

The trouble with the cloths is that they aren't that easy to get perfectly smooth so I often have annoying wrinkles, creases or shadows that I just can't light out. It's not an issue on the black cloth, and I can get rid of them during the keying process on the green cloth, but it's very noticeable on the white cloth.

What I have found to work really well is to apply the Glow effect in moderation. That seems to even out the white really nicely without impacting on the talent (unless he or she is an albino wearing a white shirt and trousers, in which case they would disappear every time they blinked).

Thanks for the tips.
I have no background yet. I was considering buying a white paper background...

Use the anchor creation tool from pan and crop to cut out the subject. Then key in a white background.

Great video btw.