[Warning: This article contains spoilers from Thursday's episode of Grey's Anatomy. Read at your own risk!]

Thursday's episode of Grey's Anatomy marked the directorial debut of star Ellen Pompeo, but the real MVP was Kelly McCreary, whose character Maggie Pierce experienced the devastating loss of her mother Diane (LaTanya Richardson Jackson).

After learning the truth about her mother's inflammatory breast cancer in last week's episode, Maggie goes into attack mode, devouring all kinds of research about the disease and its various treatments. Against pretty much all of her colleagues' advice, Maggie puts her mother on an aggressive regimen, eventually securing her a place in a clinical trial for an experimental treatment — and firing Meredith (Pompeo) as her mother's surgeon in the process.

The episode spans about a month, during which time the trial takes a devastating toll on Mrs. Pierce's body, until one morning she wakes up feeling unusually energetic and cheerful. She encourages Maggie to invite her friends over for dinner that night, and while everyone's having a great time laughing and joking around the dinner table, Mrs. Pierce has a coughing fit and is rushed to the hospital, where the doctors discover that the cancer has spread. She opts to forego additional treatment and dies a short while later, with Maggie by her bedside, painting her nails.

Grey's Anatomy stars reveal their (other) favorite TV doctors

TVGuide.com chatted with McCreary about what this loss means for Maggie, why it was helpful to have her co-star Pompeo directing the episode, and whether Maggie will heed her mother's advice going forward.

Maggie goes through such a range of emotions in this episode. What was your reaction when you first got the script, and what did you do to prepare?
Kelly McCreary:
I knew that this storyline was coming down the pike. I honestly struggled a lot more with just the preceding episodes, because it was like, ahh! Why does she have to be like this? Doesn't she know? I had the same reaction the audience had. I was like, "Why does she have to say this? That's so mean!" That stuff was harder, and I think just added so many layers to what Maggie was going through as she was trying to guide her mother through this journey. There was just all of the regret for having behaved that way and for having had that misunderstanding, and the fear of the worst happening. There were just a million layers to play in every single scene, and that is simply an actor's dream. If it's not already there in the writing, that's exactly what you try to create for yourself, is the internal conflict, the inner life of a character. That was all there already, so that was thrilling. To prepare, I did a lot of reading about IBC and the treatments, just the way that Maggie goes through trying to find out about what other patients had experienced. In other words, I did research in the same way that Maggie did as she was trying to figure out how to treat her mother.

How was it having Ellen as a director on this episode?
Ellen is my castmate. I've been directed by other castmates, too, but early on, she asked me my thoughts, and I thought about what themes really jumped out to me that I thought she could think about as she was approaching her work. And isolation was a big part of it. To be able to make any contributions to a director's process was kind of exciting, before we actually get to the shooting of it, which is always very collaborative. So, it was a really, really incredible experience. And then, when we were actually shooting it, I became hyper aware of how many people right there in that room with me, let alone out in the world and in the Grey's Anatomy audience, probably have experienced something similar to this, the loss of a loved one. And it became even more urgent to keep working to find every possible shred of emotional truth in this very specific story so that it would be universal, so that it could feel universal to the experience of losing a loved one.

Maggie's dealt with plenty of patients but has never had such a personal tie to a case before. What is this going to do to her going forward, both personally and as a doctor?
I think, first of all, the grieving will come in waves. She's sort of chipper and cheerful by nature, and she's going to really try to get back on her feet. But also, she's sensible. She knows that grief is a real thing, and that she will have to take a break from work and treat herself with a little bit of kindness, and lean on her sisters. So, all of that will be true. And then, going forward beyond that, I hope — we have yet to see, but I really hope she takes her mom's final words to heart. Even though I think Maggie is super fun and has a lovely life.[Laughs] Maybe she could take her mom's advice and sort of let her hair down a little bit.

Yeah, her mom tells her to start being "a little lazy" and "a little slutty." Will we see that side of her?
What would it look like?! What would it look like to have a lazy cardiothoracic surgeon, though, really? I don't know if I really want to see that. [Laughs]Falling down on the job in the OR?

She accidentally kills a patient or something.
McCreary: [Laughs]Right. She decided to nap instead.

Grey's Anatomy adds Matthew Morrison as... Jo's husband?

Did you know that Maggie's mother was going to be brought in for this storyline specifically? Were you disappointed we didn't get to see more of their relationship?
I did know in advance that this would be Maggie's storyline. And then LaTanya was cast in the role, and she is so terrific that, in the last days of shooting the episode, we were talking about, like, "Maybe Diane has an evil twin sister." [Laughs] Trying to plot ways to bring LaTanya back. And, yeah, of course, it would have been great for the audience to see more of Diane and Maggie's relationship, but I think the gift of having the character come on for any length of time is that, for me as an actor, now I have even more that I can imagine about what their relationship is like. I have even more stuff to add to my personal back story cache, and to figure out how to play in every scene going forward.

There was kind of a role reversal, with Maggie taking charge of her mother's treatment. We learn a lot about Maggie in this episode. Was there anything that surprised you?
I think the side of her that the audience [saw] is ... the reason why she is where she is today. She is a very, very young head of cardiothoracic surgery with many credentials. And she's well known in her field at a very young age. It takes a lot of determination. It takes a lot of brilliance. It takes a tenacity and a stubbornness to achieve those things. I think that that's what we see coming out when we're watching her try to deal with managing her mother's treatment. Stubbornness and tenacity and determination are really great qualities, but there's two sides to every coin, and I think we're seeing the other side of them.

It was so strange to see Bailey (Chandra Wilson) and Webber (James Pickens Jr.) almost intimidated by her.
Oh, yeah. It's totally like, they care about her, but they're also like, this chick will rip our heads off if we don't do what she says. [Laughs] And that is why Maggie is where she is today.

Riggs (Martin Henderson) is really sweet to her. Does that hug rekindle her feelings for him?
No. That hug is the first time she lets somebody hug her. While she's been pushing everyone away, it's the first time that her sort of façade of having it all together breaks down. Well, it's broken down. Everyone else can see it. [But] it's the first time that she thinks that she's letting it break down. And I think, really, that's what that moment is about. She has, for the first time, lost control, and I think it has less to do with it being Nathan there in the room with her, and more to do with how intense the situation has become with her mother's health.

Meredith and Riggs have decided to put their relationship on hold again for the time being, but something tells me that's not going to last. How is Maggie going to react when she finds out Meredith has kept this from her? It seems like her feelings about Riggs aren't as strong as they once were, but also Meredith's been lying to her for longer at this point.
It's a longer period of time, and it's also a different set of circumstances. Maggie's mom has died now. So, the question of whether Meredith should have told her becomes less about the fact that they are together and Maggie wants him. It's less about the fact that Meredith took something that Maggie wanted, and more about... something else. ... It's not about Meredith taking the boy Maggie had a crush on. Maggie has experienced something that makes that sort of trivial at this point. I don't want to give too much away about that storyline, because it is one of the major questions left to be answered this season.

Maggie and Meredith butt heads in this episode but seem to reach an understanding at the end. How does this experience affect them?
I think that it will definitely bring her closer to Meredith — and Amelia, too, with the loss of her father. Even though the circumstances are completely different, they share something that they didn't share before, and it'll bring a new level of understanding and closeness, I think, to those relationships.

Grey's Anatomy airs Thursdays at 8/7c on ABC.