Gossips have speculated for years that the pair's explicit love scene in the 1973 film actually showed unsimulated sex, and now former Variety editor Peter Bart has written about his time onset in his new book, Infamous Players: A Tale of Movies, the Mob, (and Sex).
He was given access to the shoot in his role as a Paramount Pictures executive and claims to have seen Sutherland and Christie filming the thriller's bedroom scene - and he's adamant they weren't acting.
Bart writes in the excerpt obtained by the Hollywood Reporter, "It was clear to me they were no longer simply acting: they were f**king on camera."
Sutherland and Christie were rumoured to have enjoyed an onset affair and the actress' then-boyfriend Warren Beatty was reportedly so upset about the saucy scene, he demanded movie bosses cut the footage.
The film's producer Michael Deeley spoke out in 2008 in a bid to quash rumours the sex scene was real, saying, "There (wasn't) any truth in the idea that Donald Sutherland and Julie Christie were actually having sex, they were not, they were acting."