"I don't envy your job," says Jason Isaacs, "because it's impossible to describe this show."
At the time of our interview, I've watched the first half of the second season of Netflix's The OA, and it's as beguiling and mystifying as ever. It's also completely insane. The first four episodes alone feature parallel dimensions, apparitions in mirrors, and a murderous talking octopus – and that only scratches the surface of the sheer number of mind-bending twists this show delivers. On paper, Isaacs knows it sounds crazy, but he has full faith in its co-creators, Brit Marling and Zal Batmanglij.
"The more outrageous, the more audacious the things they come up with are, the more I'm convinced they're going to get away with it," he says.
"I have no idea where they get their ideas from. They're genuinely the most refreshing and original writers and creators I've ever come across in 30 years."
The OA Part II returns with Prairie Johnson – aka the OA – waking up in another dimension, having successfully "jumped" in the season one finale. The story shifts to an alternate version of San Francisco, where OA is drawn into a mystery involving a missing girl, a sinister game developed by a Silicon Valley entrepreneur, and a mysterious house full of puzzles and hallucinations.
Isaacs' character, Hap, wakes in this new dimension as Dr Percy, now a successful, respected doctor devoting his life's work to metaphysical concepts and the exploration of the human mind. "He arrives somewhere where he gets a lot of validation; he gets a lot of status," says Isaacs. "I work in this odd world where some people are anointed and given god-like statuses, and you watch how much it changes people despite themselves.
"The potential in Hap for kindness and love and generosity – it just had no place in the work that he was doing in season one and it has some place here. He tries very hard to hold on to, as we all do, the things that he thinks he's aiming for and wants to be aiming for. He just is sometimes distracted by affairs of the heart, or shards of humanity that stick out."
Despite being surprise-dropped with little fanfare on Netflix in late 2016, The OA rapidly found an obsessive fanbase all over the world. Known by a generation for playing Lucius Malfoy in the Harry Potter films, Isaacs is familiar with dealing with hardcore fans – but with this show, he's never seen anything so original touch people so quickly.
"I get asked in the streets so many times, I can't tell you," he says. "Everyone wants to know every detail about part one – they want to know which bits were true, which bits weren't, they wanted to run their fan theories by me. The real truth is, they don't want any of the answers, because the answers are told in the story, and some actor telling you in the supermarket isn't going to be satisfying at all."
It helps that Isaacs, as a viewer, found himself completely sideswiped by the series. "I'm probably as big a fan of the show as anybody else," he says. "When I had to do the first lot of press for it in England, I was sent a link the night before I had a full day of interviews, and I thought, 'I better watch it for two minutes just to get a sense of the flavour of it'.
"I clicked on it at nine o'clock at night, and I watched it straight through to five o'clock in the morning with no break. I was kind of devastated by the end – I was laughing and crying and shaking like I'd had some sort of cult religious experience."
Isaacs was touched by what so many others were touched by – that underneath the story's fantastical elements, there lay a carefully mapped, character-driven story about broken people finding connection in each other. It's a testament to Marling and Batmanglij's writing, says Isaacs, but also a mark of the power of good storytelling.
"I remember my first day on Harry Potter, thinking, 'Well I'm in this fun thing now where we dress up as wizards and play pretend'," he says. "My very first scene was with Richard Harris, and he absolutely nailed me, and in the very first instance, I was jolted back to reality and realised that whatever world you're portraying, you're trying to make it utterly real.
"All of your inner fears and insecurities, and your superiority complex, whatever it is, are every bit as human, no matter what situation you're in. So this isn't any different to any other acting – it's just that the story is possibly richer and more emotional."
As the show's second season explores the possibility of alternate realities, Isaacs says that's not something he's even comfortable giving too much thought to. "I'm slightly terrified of there being things beyond science because I've always been in horror films and fantasy films, and having delved into it and met enough people, I do think there are more things than heaven and earth," he says.
"But I don't particularly want to come across them since the very thought gives me goosebumps... I've got enough to worry about in this world," he says.
Who: Jason Isaacs
What: The OA Part II
When: Streaming now