To differentiate 2001 from the "flying saucer pictures" that owned the sci-fi genre at the time, Stanley Kubrick planned to begin the movie by showing interviews with 21 real-world scientists about their predictions for the future and the likelihood of extra-terrestrial life.
This prologue was dropped for length, and the footage has yet to be located, but recently discovered transcripts of the interviews will be published next month in the UK. The Independent reports that Carl Sagan was on the list to be interviewed, but was cut when he demanded editorial control and a percentage of the gross. millions and millions.
2001: The secrets of Kubrick's classic [independent.co.uk via...um]
Are We Alone?: The Stanley Kubrick Extraterrestrial Intelligence Interviews [amazon.co.uk]