Alyssa Milano and Holly Marie Combs played witch sisters Phoebe and Piper for eight seasons on Charmed

Grey’s Anatomy showrunner Krista Vernoff was overjoyed to reunite with former colleagues and Charmed stars Alyssa Milano and Holly Marie Combs for Thursday night’s episode — so much so that she decided they should channel their witchy characters, sisters Phoebe and Piper.

“Our goal was to wink at Charmed while still making an episode of Grey’s Anatomy,” Vernoff, who worked on seasons 3-6 of Charmed, tells PEOPLE exclusively. “Holly’s character is more serious, a lawyer. Alyssa’s is more free-spirited. They bicker a lot, but underneath it, they love each other.”

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The fantasy series, which went off the air in 2006, holds a special place in Vernoff’s heart because it was her second television job.

“It was incredible to see Holly and Alyssa work together again,” Vernof says. “There’s a playfulness when they work together — a joyful, sibling-esque quality that just instantly came back to life. They are powerful women, forces of nature, and I so deeply admire their political activism. It was nice to see the joy and fun they brought out in each other, too, as if no time had passed.”

Image zoom Krista Vernoff

“I learned so much there,” she adds. “And it payed off in more ways than one because Shonda [Rhimes] was a Charmed fan — and we talked about it a lot in my job interview for Grey’s!”

In an exclusive sneak peek of Thursday’s episode, Combs and Milano — who play the sisters of a patient who may be brain-dead after falling at a construction site — must decide whether or not to keep their other sister alive.

“What are you saying?” Combs’ character asks Dr. Richard Webber (James Pickens Jr.).

“She fell about 30 feet,” Richards says. “She fractured her skull. We’ve done everything we could, but I’m afraid there are no signs of brain activity.”

Image zoom Holly Marie Combs and Alyssa Milano ABC

Combs (who just got married!) and Milano played witch sisters with Shannen Doherty in Charmed’s first three seasons, and then alongside Rose McGowan in the show’s second half.

With a shocking season 16 premiere that included a breakup, a pregnancy, and a proposal, Vernoff guarantees that Grey‘s fans won’t be disappointed moving forward.

“They can expect even more twists and turns,” she says. “We are shaking it all up in season 16!”