The skeleton found by Tuco inside the wrong coffin at Sad Hill cemetery, was a real human skeleton. A deceased Spanish actress wrote in her will she wanted to act even after her death.
The Good, the Bad and the Ugly (1966)
In order to make their forced perspective shots work with non-static cameras special rigs were built to move the camera and the actor simultaneously.
Lord of the Rings: Fellowship of the Ring (2001)