The actress, 54, first joined the long-running medical drama in the season 1 finale as Derek Shepherd (Patrick Dempsey)'s estranged wife, Dr. Addison Montogomery. She stayed on as the neonatal surgeon for several seasons before leading the spinoff series Private Practice, which aired from 2007 to 2013.
Last week, however, Addison made a return to Grey-Sloan Memorial and is also set to appear on Thursday's episode.
During a panel Monday evening, Walsh told PEOPLE and other reporters that reuniting with her former costars on set was both "intense" and "very emotional."
"All the feels," she said. "It was really beautiful. We reunited at the COVID test tent, and hugged each other with our masks on, and then went to go act with our masks on."
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"It was just very emotional," she continued. "Like going back to your hometown and your family and your first best job ever all at once. It was a lot, it was great."
Walsh said seeing Ellen Pompeo, Chandra Wilson and James Pickens Jr. — the only original Grey's stars that have remained on for all 18 seasons of the series — was particularly "meaningful and powerful."
"I just love them so much," she said, adding that it was also special to reunite onscreen with Caterina Scorsone, who starred in Private Practice with her.
For Thursday's episode, Walsh teased that fans will get to see a lot more of the relationship between Addison and Scorsone's Amelia Shepherd — the late Derek Shepherd's younger sister.
"The second episode is a little more contained, and you really see Addison and Amelia's dynamic play out and their reconnecting," she said. "That was a total delight for me and I think fans will enjoy that as well because it's a completely different kind of episode than the first one, but intensely intimate and satisfying."
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As for whether she will be staying beyond these two episodes, Walsh said "we'll see."
"Right now we're just doing a few episodes and we'll see what happens," she said. "Nobody knows what the future holds — I think the last year or so is evidence of that — but for now this is what we've got planned, just to have Addison pop in."
Grey's Anatomy airs Thursdays (9 p.m. ET/PT) on ABC.