Ever wondered how movie directors achieve those intimate shots of actors cosying up to apes? Sigourney Weaver managed several close encounters in Gorillas in the Mist, so did Rene Russo in Buddy, and Christopher Lambert made perfect contact in Greystoke: The Legend of Tarzan, Lord of the Apes. Adult gorillas and chimpanzees have a talent for carnage, and prized actors frolicking with real apes is an insurer's nightmare. Enter ape actor, Peter Elliott. For it was Elliott, in animated ape costume, who got to gaze adoringly at Weaver and Russo. Elliott has also choreographed hominids and played one in Jean-Jacques Annaud's 1981 film Quest for Fire.

Before Hugh Hudson's Greystoke in 1979, the apes used in movies were all youngsters. Hudson wanted his chimps to be the size of adult chimps, yet he could not risk the lives of the cast and crew. Auditions for actors to play chimps began and lots of theatrical miming and cliched rib scratching ensued, much to Hudson's dismay. But Elliott, then aged 22 and not long graduated from "method" drama school East 15, had the good sense to visit London Zoo to observe the chimpanzees prior to auditioning.

Elliott's audition had the necessary realism, but with only one actor able to give any depth to his chimp character it became clear to Hudson and the producers that if they didn't want the film to be a comedy, they'd better go back to the story board. Elliott was invited to LA for a further audition, telling his family he'd be gone for a week. But after a meteoric promotion to head of research and development for Greystoke, Elliott remained in the US for another two years. His R&D job took him to Oklahoma to study chimpanzees.

Primatologist Roger Fouts, who went on work with Elliott on Greystoke as a consultant, had worked on the chimp language study Project Washoe. Washoe was a chimp caught in the wild and estimated to be between eight and 14 months old when she arrived in Nevada. She spent her early years living in a house, treated as a surrogate human child and taught by scientists to communicate in American Sign Language. She was reported to combine signs in original, meaningful ways, for example she named the refrigerator, "open food drink", when the psychologists had always referred to it as the "cold box".

When Project Washoe ended in 1970, Washoe and Fouts transferred to Oklahoma's Primate Research Centre. By 1977 Fouts was working on Greystoke and for the purposes of R&D he allowed Elliott to go in with the chimps. "I immediately learned that chimpanzees were emotionally unstable, highly intelligent and incredibly strong," Elliott told me.

"To say that I was amazed by Washoe, Nim, Ali and Mac and the other chimps would be an understatement. I tried to learn sign language, but Washoe realised I didn't understand so she signed extra slowly for me. I remember Washoe was given a purple ball, but she didn't know the sign for purple, so she signed, 'What colour, different red?' On another occasion she signed that she wanted to eat oranges, I told her there were none, then she signed, 'Get car and get some!'

"But I didn't want to sign, I didn't want to make them become human, I wanted to learn from them how to speak chimp. So I started mimicking their behaviour. At Oklahoma I learned all the basic chimp sounds and the five basic faces: the 'concentration face', where the lips are together and the top lip is pushed out with air; the 'pant hoot face', with puckered lips, that's the most famous chimp expression; the aggressive, wide open mouthed 'attack face'; the 'play face', top lip down mouth open; and the 'scream fear grin', mouth shut, with teeth on show."

Elliott told me more about chimpanzee psychology. "My research at Oklahoma was just the beginning of my long journey to understand that great apes and the ape species are all very different. Chimps have high curiosity mixed with a short attention span. They have a whole rhythm to their bodies and they are constantly vigilant, always checking out the social world. They can be deceptive, but they tend to be clear communicators.

"Chimps live in the moment. I was rough-housed by the chimps many times. I used the 'scream fear grin' when I needed to. They frequently tricked me into giving them things they shouldn't have. I learned their body language and communicated with them on their terms and eventually became a player in their hierarchy. One of the most striking anatomical differences between chimps and us is their superior upper body strength: no human can climb the way a chimp can. They were much more gentle with me than they were with each other. I groomed the chimps, ate with the chimps and they accepted me."

In Greystoke, Elliott played Tarzan's adoptive, chimp father, "Silverbeard". But in learning how to be a chimp during his research, Elliott was like Tarzan in reverse. The fictional character undergoes a culturally civilising metamorphosis, leaving his chimpanzee family behind, wherease in real life Elliott was regressing psychologically, becoming more animal and less human. It was time to return to human civilisation.

"Looking back, it was pretty risky letting me go in with them like that. My approach of imitating the chimp's behaviour and becoming intimately physical was completely different from the trainee psychology students' methods. The chimps accepted me on their own terms and the students appeared jealous of the progress I made in communicating with them. I certainly had a lot more fun with the chimps than the scientists did."