I think Zombie84 over in the General Star Wars Discussion part of the site hit it on the head:

It's how the scene would have looked if it was released with the film in 1983, but the only new footage shot was of C-3PO and either Mark Hamill with Artoo, or a stand-in for Mark. The Vader stuff was culled from deleted ESB and ROTJ footage, and lines that hadn't been used in other scenes. 3PO was composited in later over the footage of Luke and Artoo, and the scene was in the cut that John Williams scored to, which is why there's music for it.