Depends on what version of the player you're exporting to and what kind of background you want to show the person over.

Generally the most robust solution is to use Flash 8 video with an alpha channel. To get an alpha channel, you do what they do in the movies -- shoot the actor in front of a blue or green screen, then use video editing software to remove that background. If your video software supports it, you can export that file directly to .flv, or save it in a video format that supports alpha channel and use a Flash video encoder to convert it to .flv.