Kathryn Hahn on getting serious with Paul Rudd and Will Ferrell in The Shrink Next Door

Of course, when she's not wearing a mask, Kathryn Hahn is one of the most recognisable actors in Hollywood. After landing a regular role in NBC crime drama Crossing Jordan back in 2001, Hahn went onto steal the show as Kate Hudson's weepy best friend in How to Lose a Guy in 10 Days, Adam Scott's hilariously frustrated wife in Step Brothers and an over-excitable suburban mum in We're the Millers.

Between flexing her comedy muscles in shows like Parks and Recreation, Central Park and Mrs. Fletcher, and embracing dramatic roles in heavy-hitting titles Transparent and Revolutionary Road, Hahn's resume is an extensive list of high-profile projects and it's about to get even longer thanks to her upcoming roles in Knives Out 2 and Marvel spin-off Agatha: House of Harkness.

Despite her unsurprisingly busy schedule, Hahn has found some time to chat about her latest series, Apple TV Plus drama The Shrink Next Door, taking a break from her post-Thanksgiving workout to pick up the phone. Unfortunately, I'm sniffling away on the other end, suffering with a heavy cold and sounding like High Wycombe's answer to Marge Simpson. If anyone were to be Vitamin C in human form though, it would be the incredibly bubbly Hahn, who prescribes "steam showers and eucalyptus" before we move onto her role in the eight-part series, which premiered back in November and is currently halfway through its run.

While you'd be forgiven for expecting The Shrink Next Door to be a laugh-a-minute riot with Hahn starring alongside comedy titans Will Ferrell and Paul Rudd, the psychological drama is unsettlingly dark – especially when you learn it's based on real events.

Adapted from Joe Nocera's factual podcast of the same name, the black comedy stars Ferrell as Marty Markowitz, an anxious single man who becomes a patient of Dr Isaac 'Ike' Herschkopf (Rudd) after struggling with the loss of his elderly parents and the inheritance of the family business. While Dr Ike's methods seem slightly unorthodox at first, he manages to slowly implant himself into Marty's life and alienate him from his only remaining family, his sister Phyllis (Hahn), before taking $3.2 million (£2.4m) from him over the course of 27 years.

Hahn stars as Marty's protective sister Phyllis, a mother of two in the midst of a divorce who suspects Ike may be taking advantage of her brother early on in the series. "I had heard the podcast before I even knew it was going to be made into a series," she says. "I think like so many people, my jaw was on the ground. That is such a sacred relationship – you give someone so much of yourself and there's so much faith and trust."

While in this real-life instance, which lasted from 1982 until Markowitz cut Herschkopf off in 2010, the psychiatrist severely abused his power and crossed several professional boundaries, Hahn stresses that she's a huge proponent of therapy.

"Therapy is amazing," she says. "I think it's such an invaluable part of being a human being. Growing up in Ohio, in Cleveland, that was something that was just not regularly offered to us as an option, to talk about feelings.

"But the idea of that relationship taking a turn, even though he happened in our version of this story to be a very successful doctor to many, many people, he just saw something in Marty that was easy to take advantage of."

Hahn was drawn to the role of Phyllis after finding herself in a similar position with a dear friend. "Our friendship has dissolved over – of course everything is subjective and who knows, but in my mind – what seems to be a relationship with somebody else that I just couldn't quite get along with.

"I felt that very deeply, that sense of loss and longing. This person was like a sibling to me and so I could really connect on a personal level to Phyllis's pain, and just the unbelievable amount of frustration and anger, just like a child doing whatever they could to get his attention."

Hahn was able to get in touch with the real-life Phyllis before filming began, speaking to her over Zoom and hearing her side of the story. "By the time I met her at the actual premiere, we basically leapt over theatre seats to get to each other to hug each other. We were so excited to be in the same room.

While the actual Phyllis and her brother were estranged for decades after Herschkopf convinced him to stop all communication with her, they managed to reconcile in the 2010s after Markowitz reported the therapist to the New York State Department. "I really have such admiration for her openness of heart after all that time on the idea of forgiveness. On the idea of not living a life of resentment or bitterness or anger. She found her own life during all those years and her own family and yet was still able to let him in, which is pretty remarkable."

The Shrink Next Door certainly isn't the first project in which Hahn has worked with Rudd and Ferrell, with the trio all appearing in Anchorman while sharing various credits such as Our Idiot Brother and Step Brothers. However, Hahn got to see a whole new side of her co-stars in the drama.

"I think they're both such extraordinary actors," she gushes, revealing that she hadn't previously "connected on this level" with Ferrell before The Shrink Next Door.

"In his movies, I was always kind of bit parts or smaller, supporting [roles] and to be able to connect with him on this level in this role, I'll just hold to my heart so dearly," she says. "He was really available. It was pretty serious business. Some of those scenes were really – in the best way – difficult for us, especially when it got to the fighting because we have such a fondness for each other."

As for Rudd, Hahn describes one particular moment in which Phyllis goes to Ike's office to confront him about the hold he has over her brother, and finds herself opening up to him as well. "Paul and I had one delicious scene that was like a short film. We got to go through all the levels together and we have done so many other things together but this was just a whole different burden. It made me want to act with him so much more."

There's a possibility Hahn won't have to wait too much longer for that to happen, with both her and Rudd now firm members of the Marvel Cinematic Universe. The age-defying Rudd joined the global superhero franchise as Ant-Man back in 2015 and has since appeared in three Marvel films, with Ant-Man and the Wasp: Quantumania on its way. Meanwhile, last month, Marvel confirmed that Hahn would be starring in a stand-alone spin-off series titled House of Harkness – although there's not a huge amount she's allowed to say about the upcoming series.

"I have to tell you, even hearing the name of the show was a surprise to me and I love it so much," she says. "They keep it really close. I'm very excited to meet with [the producers] and hear what they're cooking up because I know it's going to be fabulous."

News of the spin-off came as a complete surprise to Hahn, who admits she had never heard of Agatha Harkness before working on WandaVision. "You'd have to be deep-diving Stan Lee, comic book aficionado to know Agatha Harkness from the comics. I had never heard of her before. So the fact that this is happening is just a testament to the incredible writing, the weird timing.

"I just remember staring at the icon that said Agatha: House of Harkness and just giggling to myself because it's so bananas. It's so meta that WandaVision was a show about sitcoms and of course, it's going to have a spin-off. It just makes total sense in a very meta way."

While she hasn't had many fans singing the ear worm that is Agatha All Along back to her due to the pandemic, the Marvel star says that she did receive a birthday book made of letters from fans. "Someone had sent through my beautiful hairstylist a book of thank you letters from all these fans because that's the way to communicate with me as I have no social media and it meant so much to me.

"I burst into tears when I was reading it. I'll hold onto it for the rest of my life. It was so tender."

With Hahn taking the Marvel Cinematic Universe by storm, perhaps it's time for her Shrink Next Door co-star Ferrell to become the last of the trio to become a superhero? "Oh my God!" she keenly answers. "I think they would be lucky to have him. He's so fabulous in anything he does."

The Shrink Next Door is available to stream on Apple TV Plus. You can see The Shrink Next Door cast in full here. If you’re looking for more to watch check out our TV Guide.