SPOILER ALERT: Do not read if you have not yet watched the sixth season of “Orange is the New Black,” streaming on Netflix.

While season six of “Orange Is the New Black” tackles an assortment of timely issues — immigration detention centers, opioid addiction, the Black Lives Matter movement — at its heart is a tale of two sisters, Barbara and Carol Denning.

Imagine “Frozen’s” Anna and Elsa as homicidal OG jailbirds whose kingdoms are rival cellblocks inside the maximum-security prison where the ladies of Litchfield get locked up after last season’s prison riot. Sentenced to life following the murder of their kid sister (compounded with drug-dealing charges), the siblings-turned-archenemies spend the better part of thirteen episodes trying to off each another.

Mackenzie Phillips —- best known for her breakout role in the 1973 movie “American Graffiti” followed by the 1975 sitcom “One Day at a Time” —- plays the modern day version of Barb, who eventually gets sober but never gets over her decades-long resentment for Carol.

When Mack, as she’s known to friends isn’t acting, she works at West Hollywood’s Breathe Life Healing Center, where she’s a group facilitator and director of referral relations for the facility that treats substance abuse, eating disorders and complex trauma.

Here, she talks with a colleague from that facility for Variety about returning to acting for two Netflix projects, the challenges of portraying addiction on-screen and how she handled Barb’s demise.

As fans of the “One Day at a Time” reboot already know, “Orange Is the New Black” isn’t your Netflix debut. How were you lured out of semi-retirement?

When I started working in treatment about six years ago, I basically said to my agent, “You know, don’t submit me for a bunch of stuff. I’m doing something different. That doesn’t mean I will never act again, but just don’t.” And the first thing was Norman Lear calling me and asking me if would play a therapist on the new “One Day at a Time.” And of course, when Norman Lear calls, you say yes because he’s Norman. And I’ve loved him my whole life.

How did you land the part of Barbara Denning on “Orange is the New Black”?

I was at a FADAP conference in Baltimore last year, and my agent called and she said, “I know [what] you said, but ‘Orange is the New Black’ called.” And I was like, “Oh, my God.” So I sat in the hotel room and [Breathe’s Clinical Director] Kathleen [Murphy] directed the audition. I put myself on tape and I dropboxed it to my agent, who sent it to “Orange is the New Black.” And the next day I’m leaving Baltimore and I’m standing in the line at security and my agent calls and says, “You’re hired.” I mean, it was a lark. I never thought I would get the job.

What was your initial reaction to the material they sent you?

I wasn’t sure what I was reading. It was [about] two sisters, but it turned out I was reading from the flashback [scene] — the younger sisters. They didn’t send me any material that I actually ended up doing. Maybe they hadn’t written it yet or maybe they didn’t want to give too much away. I don’t really know.

Did they tell you anything about the character in advance?

I read the breakdown. It was like, “Barb is a hardened criminal who has been imprisoned for life.” I knew it was some kind of hardcore woman. And I knew that she was a drug kingpin in the prison before I accepted the job.

Barb kills her younger sister without remorse and she plans to kill her other sister in prison. Would you say that she’s a sociopath?

I wouldn’t say that. I would say that she has a lot of issues. She’s really angry. And when you first meet Barb, you can tell that she’s been dipping into her own stash and she’s strung out and losing control of what she started in this prison years ago. You’re watching her disintegrate into homicidal madness.

How did you prepare for this role?

I really just put myself in a world where this was possible. … I’m a very easygoing, happy-go-lucky person. I’m not particularly dark or brooding or angry. And so it’s sort of the antithesis of who I am — and who I was in my active addiction. I just considered what it would be like to be someone who had no options — someone who hadn’t gotten the care they needed for a substance use disorder. You know, I’ve had access to the best treatment for myself over the years, and it just makes me think about all the people who are struggling: women in prison, women everywhere who don’t have the resources or the good insurance to access the kind of care that I’ve been able to access over the years.

What is your approach to acting?

I wanted to discover how I felt when I was doing it with the other actors instead of deciding how I was going to feel before I even got on set. I think that’s a trap that I’ve fallen into before where I make all my decisions as I’m memorizing before I even speak the lines out loud to another human. I didn’t want to do that with this. I just wanted to walk in and see what happened. And it was the kind of set where you could do that.

How did you get inside the head of this character every day on set?

The bleak atmosphere — the cells and the clothing and the no makeup — it really lends itself into going deeper and doing that kind of work. There’s no one following you around touching up your concealer. You know, I have really white teeth. And so I wore an Invisalign that was all yellow and brown that they made to fit [over] my teeth. When you look at yourself and you have no makeup on, and all of your bumps and warts or whatever are there for everybody to see, it’s bleak. I got a feeling of the hopelessness and the endless future that she could see for herself. Just like, “God, why would I even bother getting sober? I mean, I’m never leaving here.”

Did you improvise at all or always stick to the script?

It was a little bit of both. You had to run lines changes that you requested by the writers but a lot of that stuff in the medical ward with Natasha [Lyonne], we got to riff a little bit. Those lines were set in stone but all the different sounds and weird stuff that we did was organic.

The story arc of this doomed relationship between two incarcerated sisters seems like a metaphor for how resentments can kill us.

You’re absolutely right. For people like me who are clean and sober, we always talk about how resentments can be the number one killer — justified anger — and this really played out for Barb and Carol. This inability to let go of something that you cannot change can just destroy you in little ways and, in this case, in very big ways.

You got clean before bath salts were a thing, so how did you know what it looked like to overdose on them?

I didn’t know. And I didn’t need to get into specifics about what the drug was. This is the world of make believe, and I didn’t want to be too concrete. It was more about the process of watching Barb see things that aren’t there, and really believe that they are, because I have been there. I have done that in my own addiction. So seeing shadows or thinking that someone is coming to get you or whatever, those things are very real for people who have been through that kind of stuff. And so that certainty yet maybe isn’t real but it sure seems real, like, “There are flies fucking everywhere and they’re going to eat me.”

Were any of the scenes traumatic for you to film?

There was a scene where I had to snort fake drugs. It was in a little baggie and they wanted me to pour it onto my hand. And I was like, “No, no, no. Get me a straw and cut it at an angle.” I went back into my, “This is how it’s done, so this is how we’re going to do it.” And the first time that I actually did snort — it was vitamin B or something horrible, God, it was disgusting — was while we were rolling. And it made my eyes water. I walked away from that going, “Thank God I don’t live that way anymore.” So I wasn’t traumatized by it. I wasn’t even triggered by it. It brought me an enormous amount of gratitude.

Barb’s death is such a long, gory scene — the camera glides slowly over your body as your sister crawls away leaving a trail of blood behind her — what was it like to film that?

I was a swimmer as a kid, and also a singer, so I have a pretty good lung capacity. People were like, “How can you hold your breath for that long? My God!” I was like, “I haven’t smoked cigarettes in six years. And I’m not, you know, smoking crack.” And so the idea that I had to prepare to not breathe and not blink and have dead eyes for that long in multiple takes was really such a welcome challenge for me. I just loved it. I don’t know why but I just love being able to be still in that way and not be distracted. The camera was on a crane and it was a very long shot. I just remember thinking, “OK, this is going to be good. I’m not going to breathe. I’m not going to blink. My eyes are going to be dead. And I’m going to do it every time.” It was fun. I had the special effects makeup girl take some pictures of me with my phone while I was lying there dead, and I showed them to my son, Shane. I texted it to him and he was like, “Mom! What the hell? What is that? Stop it! I can’t even look.” I actually captioned it, “Mommy’s dead!”

Is acting therapeutic for you?

It’s something that is in my bones. It’s something that I started doing when I was 12. And I’ve done it pretty much my whole life. The first time that I ever made an income that wasn’t as an actor was when I started working in treatment. When I think about it, even being a group facilitator in treatment, you are putting on a show. You are attempting to engage a group of people. You’re trying to elicit an emotional response from the people you’re with. So I get to do that at Breathe, but it’s something that I love to do. I wouldn’t go as far as to say that it’s therapeutic, but it is very rewarding. It makes me feel good.