How to make a transparent background for shape/person?

[kade119]

i see these flash videos of people speaking w/ no background.. how do i go about creating this effect?

[atomic]

http://www.adobe.com/devnet/flash/articles/alpha_video_02.html

http://www.peachpit.com/articles/article.aspx?p=669597&seqNum=2&rl=1