The dangerous and often unflattering nature of filmmaking means that actors are frequently switched out with doubles, be it a stunt double specially trained to execute a tricky manoeuvre, or a body double showing more skin than the A-lister feels comfortable doing.
These are the two main reasons that doubles are employed on movie sets, but there are countless others, whether for the sake of boring practicality - like an actor not being available for reshoots - or to achieve a shot which literally cannot be completed without the actor appearing to be in two places at once.
These 10 films, from mega-budget blockbusters to small-scale art-house films, all made wildly unexpected use of doubles for myriad reasons, yet in each case you'd certainly never have guessed.
Though we've all seen movies where stunt and body doubles are hilariously obvious - the absence of the actor's face is always a dead giveaway - these 10 examples impressively pulled one over on audiences.
Without being shown the evidence for yourself, you'd probably never have noticed...
10. Tony Stark Calls It "Quits" - Iron Man 3
Though it's no secret that the Marvel Cinematic Universe regularly uses stand-in performers to help achieve some of the digitally de-aged likenesses of its stars, Iron Man 3 features an especially sly sleight of hand where Robert Downey Jr. is concerned.
Prior to shooting the film's final scene, in which Tony Stark (Downey Jr.) returns to the remains of his house, Downey Jr. suffered a debilitating leg injury, forcing director Shane Black to complete the scene using a combination of a body double and a CGI "mask" of the actor's face.
While the VFX trickery is subtly noticeable once you're aware of it, it's pretty much imperceptible if you don't already know. You'd have no idea you were looking at a digital face pasted onto an anonymous stand-in's body. Incredible.