"The Transformers: The Movie" was ahead of curve in using celebrity voice actors. On top of Welles, the cast included Judd Nelson as young hero Hot Rod, Robert Stack as the stern Ultra Magnus, Eric Idle as the wacky Wreck-Gar, and Leonard Nimoy as Galvatron. Despite this esteemed company, Welles wasn't impressed with the experience. According to Peter Bogdanovich's book, "This Is Orson Welles," Welles recorded his lines on October 5, 1985, five days before his death. In that brief span, he still had time to bemoan the role. In the closing pages of Barbara Leaming's "Orson Welles: A Biography," Welles recounts to his interviewer:
"You know what I did this morning? I played the voice of a toy. Some terrible robot toys from Japan that changed from one thing to another. The Japanese have funded a full-length animated cartoon about the doings of these toys, which is all bad outer-space stuff. I play a planet. I menace somebody called Something-or-other. Then I'm destroyed. My plan to destroy [whoever] is thwarted and I tear myself apart on the screen."
It's easy to see why Welles felt this way. Unicron may be planet-sized, but he was also a paper-thin character. He's basically Galactus but a Transformer, complete with the herald (Galvatron filling in for the Silver Surfer). His other goal beyond planet-eating — destroying the Autobot Matrix of Leadership — also falls flat because the movie gives no reason why the talisman can destroy Unicron.
A contemporary review in the Los Angeles Times put it succinctly: "The great animated villains, like the Wicked Queen in Disney's 'Snow White,' had motivations as compelling as any live-action character. Unicron apparently destroys entire worlds because it has nothing else to do."