The prequel series, which will be executive produced by Jensen Ackles and his wife Danneel Ackles, focuses on the "epic love story" between John (Drake Rodger) and Mary (Donnelly) — the parents of Sam (Jared Padalecki) and Dean (Ackles) — as told from the perspective of Dean.
Speaking with PEOPLE exclusively, Donnelly says it's been "surreal" stepping into the iconic franchise.
"I don't know how I keep getting so lucky because everybody on The Winchesters and in the Supernatural universe, like Jensen and Jared, are the nicest people alive," she tells PEOPLE.
Matt Miller/The CW
After working on the pilot in April, Donnelly says she's more than eager to get back to New Orleans and start filming again as the show preps for its fall release.
Of course, with the Supernatural franchise having such a large and dedicated following, Donnelly admits there's an added "pressure" to working on the prequel series.
"We want to make sure that the fans are so happy with the product," she says. "We care so much about what the fans want because we know how much it means to them."
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The original Supernatural series ran from 2005 to 2020, becoming The CW's longest-running series with 15 seasons. The Winchesters will be Supernatural's first official spin-off since its conclusion after previous spin-offs, including Supernatural: Bloodlines and Wayward Sisters, were scrapped.
"It's really exciting because we get to play around with the Supernatural universe," Donnelly adds of the upcoming series. "I'm so grateful."