Can anyone enlighten me on the cover letter? Specifically when it is appropriate to use them and, more importantly, what to include in them? I know that they're an intro to your resume and should be short, but does the content depend on the occasion; say, trying to get an agent to sign you versus submitting for an audition? Thanks!

The cover letter isn't even necessary. Agents get it that you're looking for representation when they get a big envelope. They're not waiting for their x-rays.

BUT at the same time, cover letters are your chance to talk about what is NOT told by your resume or headshot. Say you saw their interview on Virtualchannelnetwork.com or Actorsnetwork, well mention it in the cover letter. "I really loved your interview..."