Doctor Who is the longest running Sci-Fi TV series of all time. The secret to its longevity is its uncanny ability to recast the lead actor every few years. But who are the people behind "The Doctor?" The Cast of Doctor Who takes you back in time to learn about the personal histories of William Hartnell, Tom Baker, Paul McGann, and the newest incarnation of The Doctor, Matt Smith, including a bonus bio of 1960's Doctor Who Movie actor Peter Cushing.