I watched All About Eve with the commentary last night, and strangely they didn't explain the rear projection at all.
A few interesting things I learned is that Joe Mankiewicz hated that because of All About Eve, an actual Sarah Siddons Award was created. Celeste Holm, the actress who played Karen, actually received the award one year, which rankled Joe.
One of the commentators was Ken Geist, who wrote a Mankiewicz biography. I wanted to find out more about Geist, and in my Googling, I found out that Geist said Eve Harrington was originally conceived as a lesbian.
Joe Mankiewicz's son, Tom Mankiewicz, was another commentator and his insights about his parents were fascinating. He said he saw his father in Bill and his mother in Margo. His mother was Rose Stradner, mostly known as a theater actress in Austria. When she married Joe Mankiewicz, she came to Hollywood and became a homemaker. It was her understudy who ended up with a successful career in Hollywood - sound familiar? Tom said his father had many affairs (for example, Joe had an affair with Judy Garland during his marriage with Rose) and Tom said he saw his parents during the fight scene between Bill and Margo on the bed on stage.
Lastly (most importantly), the third commentator was Celest Holm. While she shared very little, my favorite trivia came from her. She said that she painted the eggplant herself! She was bored waiting around set sometimes, so in between takes, she actually painted that still life of bananas and eggplant. She sounded really proud of the painting LOL.