Peter Firth, who had played 17-year-old Alan Strang on the London stage from July 1973 and on Broadway from September 1974, was actually nearly 23 when the movie version began filming in September 1976. Firth had played Alan Strang over a thousand times on stage.
Playwright Sir Peter Shaffer adapted his play for this film version and wrote the screenplay for the movie. Shaffer, who was on-set watching off camera, was horrified by the way Sidney Lumet directed the final scene in the stables, claiming he had made it like the shower scene in Psycho (1960).
The film was considered groundbreaking at the time for the amount of full frontal male nudity by Peter Firth. Firth was shown completely nude three times in the film, and the final scene in the stable is one of the longest scenes of full frontal male nudity in any mainstream film. However, Firth's penis was not allowed to be shown erect at any time.
Richard Burton still had dye in his hair from his previous picture Exorcist II: The Heretic (1977) that made his hair look orange when the light hit it. When Sidney Lumet told him, Burton sighed and said "Well, maybe if I'm good enough they won't notice."
The film's producers were very glad when the movie received an R rating, as they were worried the extensive full frontal male nudity would qualify it for an X rating. Three versions were filmed of the final scene of Peter Firth in the barn; one where he was completely naked and repeatedly shown full frontal, a second version where he was only filmed naked from the waist up for television versions, and a third version where he was not shown naked at all in case the censors would not allow the nudity at all. Sometimes when the film is shown on television the scene is heavily edited, or the picture is cropped in such a way that Firth's penis and scrotum are not shown.
Richard Burton reportedly rejected the initial choice for the role of Alan Strang, telling the producers the actor was too tall. He insisted Peter Firth, who had starred in the play with him at the Plymouth Theater on Broadway in April 1976, should be cast as Firth was about the same height as him.
The original Broadway production of "Equus" by Peter Shaffer opened at the Plymouth Theater in New York on October 24, 1974, ran for 1209 performances and won the 1975 Tony Award (New York City) for the Best Play.
On the stage, actors portrayed the horses wearing unitards and extra-large tribal horse-head masks made of paper mache, fabric and wire. In this movie, they were replaced by real horses. The play was opened up from the stage adapted to realistic settings. Some critics slammed the movie for saying that the play's spirit had been removed.
One of the film's main taglines "I am yours and you are mine" is a line of dialogue from the script. It is said during this section: "Dysart: And he? What does he say? Alan Strang: "Mine. You're mine. I am yours, and you are mine. I see you. I see you always. Everywhere. Forever. Kiss anyone, and I will see. Lie with anyone, and I will see. And you will fail, Alan. Forever and ever you will fail. You will see me, and you will fail. The Lord thy God is a jealous God."
The movie garnered Richard Burton him his seventh and final Academy Award nomination for acting. Burton never won an Oscar but it is widely believed had he lived longer, Burton would have at least been awarded an Honorary Oscar at some stage.
The picture's source Peter Shaffer play has always been controversial for its depiction of male nudity and animal cruelty against horses. Further controversy erupted in 2007 with an English stage revival of the play. It cast "Harry Potter" star Daniel Radcliffe, a then 17-year old, in the Alan Strang role, requiring Radcliffe to do nudity despite the fact that the play had been performed with nudity over a number of decades. Radcliffe stated that his nude scene was not "gratuitous" and that he wanted to portray the character as originally intended by the play's script. Radcliffe chose not to view this movie version so it wouldn't influence his interpretation.
The producers did not initially want Richard Burton to play Martin Dysart, due to his reputation for alcoholism and overacting. Media reports indicated Marlon Brando and Jack Nicholson were interested in the part. Burton was forced to make Exorcist II: The Heretic (1977), to show that he could still carry a commercially successful movie, before they would relent.
'Equus' was originally produced on the stage in London by The National Theatre of London and directed by John Dexter. It was later produced in New York by Kermit Bloomgarden and Doris Cole Abrahams, in association with Frank Milton and Doris Cole.
Colin Blakely was unable to accept a role of a Police Inspector in US TV film Spectre (1977) because he had been booked for this project. Blakely "changed costumes" for this movie as he had played the Martin Dysart (played by Richard Burton) role on the London West End stage. Instead, Blakely played Alan Strang's father.
Writer Peter Shaffer and director Sidney Lumet worked for over a year in development and pre-production preparing Shaffer's "Equus" stage-play for this film version. Lumet had agree to do the film as a favor to Shaffer.