A few body and stunt doubles are listed in The Secret Garden's credits for star Dixie Egerickx. While the use of stunt doubles in movies is well known and commonplace for obvious safety reasons, doubles are also frequently used for other reasons.
Stand-ins - who usually have similar looks to the actors - are used before filming when lighting and camera set-ups are being planned. Sometimes they work as on-camera doubles too.
Sometimes this is for scheduling reasons. In the 1950 Disney Treasure Island, young American actor Bobby Driscoll (who played Jim Hawkins) was only able to stay in Britain for six weeks of filming. A double was used for shots after his departure. Shooting doubles from a distance, from behind or shooting body parts such as hands are often used in films to save time or money (an actor could be working elsewhere simultaneously), spare an actor unnecessary work, or to pick up extra shots or create inserts.
Bobbie Koshay was Judy Garland's stand-in on The Wizard of Oz (1939) and also appeared on camera in some shots, including Dorothy's fall into the pigsty and her being lifted by the winged monkeys.
Careful coordination made Dorothy's entry into Oz more effective. During the transition from sepia to colour when Dorothy opens the house door in Oz, Koshay, seen from behind and in a sepia-coloured costume, opened the door and moved out of shot, then a normally costumed Garland walked through the doorway into Munchkinland and Technicolor.
Bert Lahr was also doubled for the Cowardly Lion's athletic entrance and running dive through glass in the Oz passageway. Jack Haley's double performed the Tin Man's fall when the Wicked Witch had him pulled into the air by invisible forces. Both actors - and Ray Bolger as the Scarecrow - were doubled for part of the climb up the mountain to the Witch's castle.
Betty Danko, Margaret Hamilton's double, made the Wicked Witch of the West's entry into Oz. Both women would be injured in fiery accidents during production: Hamilton when exiting Munchkinland was severely burned when the timing of the fire and smoke was off and Danko also suffered burns while riding a "broomstick''( a smoking pipe) that unexpectedly exploded.
Unless you have real identical twins, as in Twin Falls Idaho (1999), they must be created. Another Disney movie, The Parent Trap (1961), had Hayley Mills playing twins separated by the divorce of their parents who meet at a summer camp. While some of the scenes with two Mills on screen were created through trick photography, a double was also used for long shots and over-the-shoulder shots to lessen the time, money and effort devoted to special effects.
Occasionally, and sadly, doubling is necessary when an actor dies during production. Robert Walker died from an allergic reaction to a drug during the production of the anti-Communist melodrama My Son John (1952). Director Leo McCarey used techniques such as doubles and shots from Robert Walker's previous film, Strangers on a Train (1951) as well as an altered ending to complete the film. The methods used were quite impressive in the circumstances and for the period, though the effort did show.
Martial arts star Bruce Lee also died during the production of a film, from a cerebral edema. Shooting on Game of Death had been suspended so Lee could make Enter the Dragon (1973). Afterwards the latter film's director, Robert Clouse, finished Game of Death using a rewritten script, visual and vocal doubles and archival footage. It was released in 1978, five years after Bruce Lee's death.
Improvements in technology have meant latterday doubling can be made less obvious. Bruce Lee's son Brandon Lee was wounded making of The Crow (1994) and died during surgery. After a script rewrite, a double and digital superimposition of Lee's face enabled the film to be completed.
Similarly, when supporting actor Oliver Reed died of a heart attack during the making of Gladiator (2000), a double and digital technology were used to complete his performance.
Paul Walker died in a car accident while making Furious 7 (aka Fast & Furious 7). His brothers Caleb and Cody appeared in about 260 new shots with Paul's face superimposed on theirs. About 90 more shots were completed with outtakes or older footage of Walker.
Occasionally, and sadly, doubling is necessary when an actor dies during production.
Sometimes a replaced actor is still visible in the finished film. Kevin Spacey is visible in one wide location shot in All the Money in the World (2017) before the film cuts to a closer shot of Spacey's replacement, Christopher Plummer, in front of a green screen.
Vivien Leigh withdrew from Elephant Walk (1954) after location filming in Ceylon because of ill health. She was replaced by Elizabeth Taylor who reshot some scenes, but in long shots Leigh is still visible.
We won't count the creation of a digital double of Peter Cushing in Rogue One (2016): such posthumous resurrections - in films, in advertisements, in television - are in their own category. The ethics and desirability of such "resurrections" can be debated, of course - should we resurrect, say, Marilyn Monroe digitally for a new "performance"?