Every statement found, in order. He did not sit for the mask, was not consulted, and has no stake in it. He is a witness to one thing only: that a cast of his face was taken, and, he says, when.
Fans on a Star Trek forum describe Shatner raising the mask on Late Night with Conan O’Brien (he could not name the character) and mentioning, in videos on his own website, a Halloween party attended as Michael Myers. The earliest public acknowledgment found; a fan recollection, not a transcript.
Sources 1
In a video with his daughter he confirms the origin and says he once went trick-or-treating in the mask. The same post reports Carpenter quipping on DVD extras, “I guess I owe the success of Halloween to William Shatner,” which was not independently verified.
“I don’t believe it was Devil’s Rain as the mask was a licensed product that came out the same year so it was probably an earlier mask.”
Sources 4
“When my kids were younger, and they’d go trick-or-treating, and I would go with them, I’d wear the mask. If [people] didn’t give them candy, I’d take off the mask, and blow a kiss.” And: “All I can tell you is, they found it somewhere in a toy store or somewhere. I don’t have a piece of the film. Maybe you can negotiate it for me!” (Entertainment Weekly)
“No, the lifecast that created the mask may have been from that movie but the Star Trek mask was created & sold in 1975 and Devil’s Rain premiered in 1974. The original Halloween movie was 1978 I believe. #FactsMatter” (The film was released in 1975.)
“I don’t remember the exact moment but I thought ‘Is that a joke? Are they kidding?’” — “I don’t think I saw the movie but I saw the mask in a picture and I recognized it as the death mask that they had made for me.” — “They made a mask of my face on Star Trek so that I wouldn’t have to be available for the prosthetics that they’d put on my face to look old or evil.” — “So somewhere along the line, someone made a mask of it for Halloween and then the story is ‘Get me a mask’ said the director and this guy ran into a Halloween store and grabbed this mask, and it happened to be of me.” (FOX 32 Chicago, promoting Halloween Kills)
Grades: Established multiple independent sources incl. first-hand or documentary evidence · Reported one credible source or the filmmakers’ standard retelling · Disputed the people involved contradict each other · Myth widely repeated, no source, contradicted by first-hand accounts.