William Russell Enoch (19 November 1924 – 3 June 2024) was an English actor who performed as both Russell Enoch and William Russell. His career on stage and screen spanned over seven decades and he first achieved prominence in the title role of the television series The Adventures of Sir Lancelot (1956–1957). In 1963, he was in the original lead cast of BBC1's Doctor Who, playing the role of schoolteacher Ian Chesterton from the show's first episode until 1965.

Russell's film roles include parts in The Man Who Never Was (1956), The Great Escape (1963) and Superman (1978). On television, he appeared as Ted Sullivan in Coronation Street in 1992. In his later years, he continued his association with Doctor Who and returned as Ian for a 2022 cameo in "The Power of the Doctor", 57 years after the character left, which won him a Guinness World Record for the longest gap between TV appearances.

William Russell Enoch was born on 19 November 1924 in Sunderland, County Durham, to Eva Compston (née Pile) and Alfred James Enoch. He was educated at Wolverhampton Grammar School and Oxford University and became interested in acting at an early age. He was involved in organising entertainment during his national service in the Royal Air Force and then, after university, went into repertory theatre.

In 1963, Russell was cast in Doctor Who as science teacher Ian Chesterton, the Doctor's first male companion, appearing in all episodes of the first two seasons of the programme except the last four.

Russell was one of the four original cast members of Doctor Who, starring opposite William Hartnell as the First Doctor, Jacqueline Hill as Barbara Wright, Carole Ann Ford as Susan Foreman and later Maureen O'Brien as Vicki. His first involvement in the series took form in the pilot episode (not transmitted until 1991), which was later reshot and broadcast as the first episode of An Unearthly Child, the programme's first serial. He eventually departed, alongside Hill, in "The Planet of Decision", the final episode of The Chase, which served as the penultimate story of the second season.

It was intended that Russell would reprise the role of Ian in the 1983 serial Mawdryn Undead alongside Peter Davison as the Fifth Doctor for the twentieth season. Scheduling conflicts left him unavailable.

After leaving Coronation Street in 1992, Russell had maintained his association with Doctor Who, having lent his voice as a narrator to several of the audiobook releases of the lost 1960s episodes. He appeared in The Game, one of the continuing Doctor Who audio stories produced by Big Finish Productions. He also recorded readings for some of the CD audio adaptations of Doctor Who story novelisations originally published by Target Books.

In 1999, Russell returned to the role of Ian for the VHS release of The Crusade, of which "The Knight of Jaffa" and "The Warlords", the second and fourth episodes, respectively, are lost. He recorded several in-character scenes to camera, which helped to bridge the gaps between the existing episodes.