Jane Fonda states in the intro to the documentary that there were 45 singers and musicians involved in the recording of the song. In actuality, there were 43: Prince left almost immediately, once he saw the "leave your ego at the door" sign, and Waylon Jennings left over a dispute with the lyrics.
Band Aid organizer Bob Geldof hung a sign on the door of the recording studio which said "Leave Your Ego At The Door". Prince (notorious for both his ego and his tantrums) reportedly drove away after seeing the sign. The solo originally planned for Prince to sing was given to Huey Lewis.
"We Are the World" was an all-star ensemble which was the American response to the UK's chart-topping single "Do They Know It's Christmas?" by Band Aid in 1984, which had similarly united many of the biggest British and Irish pop stars for the cause of Ethiopian famine relief.