More than 16 years after Kate Walsh made her splashy “Grey’s Anatomy” debut as Dr. Addison Montgomery-Shepherd, the then-unmentioned estranged wife of Dr. Derek Shepherd (Patrick Dempsey), the actor is back in her old stomping grounds.

Walsh was so dynamic as Addison, ABC launched its first “Grey’s” spinoff, “Private Practice,” centered around her character. The drama, which co-starred current “Grey’s” star Caterina Scorsone, who plays Dr. Amelia Shepherd, ran from 2007-2013.

“It felt just beautiful and I was so pleased,” Walsh told reporters of fan reaction to her return to “Grey’s” during a discussion on Monday. “For years and years, there wasn’t an interview that I’ve ever done where someone hasn’t said, ‘So fans are wondering, will you ever go back to “Grey’s”?’ And so, I did.”

“I think it seems successful,” she added with a laugh. “I was very satisfied and delighted by it. And, honestly, that’s always been [the thought]: you do this for the fans. One of the great experiences of having been on ‘Grey’s’ way back in the day, when it first began, was this crazy meaning of it being critically acclaimed and being a delight to do as an actor, [while] having mass appeal and having fans just love it.”

Before returning to the long-running ABC drama, Walsh was warned by another vet who had his own recent resurrection to expect emotions to run high. She recalled running into Patrick Dempsey, who played Dr. Derek Shepherd for a decade, “randomly” on a plane from France only a month before she was set to return to the show.

“I told him I was going to come back, and he had said, ‘Oh, it’s going to be so intense for you. And so emotional, like so many feels and then closure, too, in a strange way,'” she said. “When we started that show, it was such a very specific time for all of us, and the entire cast…it really changed every single person’s life in a pretty dramatic way.”

Returning now, in Season 18, is like a “fever dream,” she said. “It was easy to show up, emotionally, and just all of a sudden get cracked open in the elevator because there’s just so much.”

But what comes next? Walsh shared a little tease of her next episode — and whether she might return.

Addison and Amelia team up

While the first hour of Addison’s return to Seattle centered more on closure about Derek’s death, the next will put her in tight quarters with her former sister-in-law and colleague.

“The second episode is a little more contained, and you really see Addison and Amelia’s dynamic play out and their reconnecting,” Walsh previewed. “That was a total delight for me, and I think fans will enjoy that, as well. It’s a completely different kind of episode than the first one, but intensely intimate and satisfying.”

After spending so much time working together on “Private Practice,” there was a concentrated effort to make sure the dynamic did not revert to what it was on that shorter-lived spinoff. “Grey’s” showrunner Krista Vernoff, Walsh said, made sure “all of the personal stuff that transpires is really still framed around these life and death moments and what’s going on with [Addison’s patient] Tova.”

“That’s the magic of ‘Grey’s’ and what Shonda [Rhimes] originally created that is still going forward and why it’s still so successful: it is all framed within this very heightened, contained, combustible space of the hospital and these very life and death moments,” Walsh explained.

For Walsh, who played Addison on “Grey’s” and “Practice” for more than 170 episodes combined, differentiating between how her character fit into the world now was something she had to think about.

When the spinoff was first launching, Walsh noted that Rhimes had wanted “Grey’s” to focus on the stories of interns working to become doctors, whereas “Practice” would show what happened when they actually got to the top of their field.

“Addison, even as fallible as she always was in her personal life, she was still a boss in ‘Private Practice,’” she said. “And they were all bosses and experts, gathered together. In ‘Grey’s,’ she’s still a mentor and a teacher, and you see that obviously with the interns in [the episode that aired last week].”

“But even with the dynamic [with] Amelia, it’s still really grounded in the Tova case and medical, and then from that, the real personal [stuff] comes out,” she continued. “It was just much more of a combustible environment, because it is actually a hospital and a teaching hospital, as opposed to a private practice [office], which is a little more lax… [more] Malibu energy, a little more chill, a little more adult. And ‘Grey’s’ just has the combustibility… There’s something magical that happens that comes out of the characters’ personal revelations, when it’s all on this ticking clock of waiting for brain scans or test results or if this patient’s going to pull through or not. There’s a heightened aspect to it that is very specifically ‘Grey’s.’”

And Addison and Amelia’s long history, both personally and professionally, could be key to getting insight into Amelia’s mindset with her recent estrangement from her ex/baby’s father, Link (Chris Carmack).

The next episode of “Grey’s” also provides a bit more insight into Addison’s life post-“Private Practice” finale, including updates on her husband and son.

“They’re still married… You hear a little more of Addison’s backstory, if you will, with Jake and Henry, particularly as it correlates to COVID and what she went through. It will be really interesting to hear where she’s been and what she’s been experiencing,” Walsh said.

Getting closure.

In the years since Addison was seen on-screen, there was a sizable change to the “Grey’s” world: Derek died.

“’How the hell are they going to deal with that?’” Walsh recalled thinking. “‘She didn’t even go to Derek’s funeral. How are we going to sort that out?’”

It took until after Addison performed the surgery in her return hour — with an assist from Meredith (Ellen Pompeo), her ex’s widow — and endured gossip from the interns before she had a moment of quiet with Meredith in the elevator. Addison broke down, lamenting that she couldn’t feel her ex there; she thought she would. (Meredith offered to let her meet his kids, which Addison took her up on.)

“I thought the writing was so beyond,” she gushed. “[Jamie Denbo] wrote it in such a sophisticated way: ‘I thought he’d be here.’ It’s so not what you’d expect. It’s so tangential, it’s so not how she’s feeling, it’s not on the nose. It was so beautiful. One of my other friends who works in post was talking about it just that specificity of the writing in that moment was so good. And I just loved it.”

Walsh was equally enthused to reunite with Pompeo. “Ellen’s just awesome, because we’re just two people who, literally up through rolling, are just gabbing, and then you stop and then go into the scene,” she said. “She’s literally telling me, ‘Oh, this is where you can get your smoothies from, this is where you should go.’ And then it’s action.”

“So, in that way, it’s awesome,” she continued. “Which may sound weird to the outside world. But I think when you’ve been doing this show for so long, or you know these characters so well…there’s this strange thing that happens when you’re just there, it’s like a little wormhole that you go into, and that’s really what makes it a fun ride. She’s been playing Meredith for so long now, and the beautiful thing is then as long as you know the lines, you just go and see what happens.”

When it came time to film the scene, “neither of us knew what really was going to happen,” Walsh recalled. “Basically what they wrote was Addison goes from laughing to crying. And then you get to interpret that and hopefully it works. I’m glad that it felt right; it felt wildly uncomfortable and weird and I think that’s exactly what it was, too. And vulnerable. Then when Meredith goes to give her a little embrace at the end, these things aren’t written… that’s the beauty of, you know, being able to do this stuff.”

Could she return?

Though “Grey’s” has notably welcomed back former performers into the fold, Walsh isn’t sure what her future on the franchise might be.

“Right now we’re just doing a few episodes and we’ll see what happens,” she said. “I was excited, though, to make the comeback to where it all began for me. The couple of episodes that we shot, there was a great response to that first one, and I think the second one people are gonna really love, too. Nobody knows what the future holds… but for now, this is what we’ve got planned: just to have Addison pop in and we’ll see what happens, what transpires.”

Grey’s Anatomy” airs Thursdays at 9 p.m. on ABC.