The actor, 50, portrays Joyce Byers in the hit Netflix series about a group of friends who investigate the supernatural forces plaguing their town.
“She’d tell them, ‘This song actually came out in 1985, and you have it in 1983,’” Harbour said. Twins Matt and Ross Duffer are 12 years Ryder’s junior, born in 1984.
“She knew all of these minute, tiny details they didn’t even know, and they had to change things in the script based on that.”
He added: “It’s just kind of epic how wild her mind is and how it goes to all these different corners.”
Ryder made her film debut in the 1986 film Lucas. After which, her acting career surged in the late 1980s with Tim Burton’s Beetlejuice, the dramedy Heathers, and the biopic of singer Jerry Lee Lewis, Great Balls of Fire.
In a separate interview with The Independent, Harbour spoke about the toll that fame and money could have on child actors.
“They are involved in something that is a minefield,” the actor said of the show’s child stars. “The popularity and the money that they’re dealing with at 12 and 13 years old is… it just makes you an adult.
“They don’t really get to have the childhood that I wish they could,” he added.
Stranger Things is available to stream on Netflix.