This one belongs in the Easter Egg Hall of Fame :)
This hidden feature is available on the disc of the theatrical version of the film, which is sold individually and is also part of the limited 2-disc edition.
Insert that disc and from the main menu, select the 'Extras' menu entry. On the next screen then highlight 'The Wicker Man Enigma' and then press the 'up' key on your remote control. This will highlight a large question mark on the screen and if you press 'Enter' now, you will be treated to some rare interview footage. It is a 25-minute 'Critics Choice' interview from the 1997 re-release of the movie, featuring actor Christopher Lee and director Robin Hardy. While the image quality of the segment is very poor, the interview itself most certainly is not!