Digital Spy caught up with Ophelia Lovibond, who plays Biba, the glamorous and troubled sibling of Matthew Goode's Rex, to find out what drew her to such a "incredibly selfish" character in such an "intense" and "dark" project.
Can you tell our readers a little bit about The Poison Tree?
"It's about a very intimate relationship between a brother and sister, Rex and Biba, when a girl, who's completely different to the two of them is kind of welcomed into their world. And then you get a very intimate insight into their world through the eyes of [this girl], Karen."
How did you and Matthew establish that strong brother and sister bond? Did you spend a lot of time together before the show?
"We fortunately already knew each other, so that was just lucky in the casting. He's a very gregarious person, very easy to talk to. You feel immediately comfortable in his company, so that side wasn't difficult. When I knew that he was playing my brother I was so, so happy. I've always wanted to work with him so I knew it would be fine, I wasn't worried about that."
And how was it working with MyAnna as well?
"When we met we basically just struck up a really good rapport. We have a very similar sense of humour, so we were kind of immediately laughing and joking about, and you know being quite tactile, and it was again just such a stroke of luck in the casting. It was just very, very easy to show that sort of intimacy when we were filming."
A big theme of the series is the lengths people will go to keep secrets. As an actress, were you able to understand why people would be driven to such extremes to keep their privacy?
"Yeah, because it's more than privacy, it's kind of the whole foundation of their lives. It's not just something that would be embarrassing if it got out, it would completely destroy everything. [Everything] they've worked to create would be undermined in a moment, and then [be] completely irretrievable.
"Although the extremes that they go to are quite, you know, intense, you could see why someone in the moment, who's completely panicked, would do anything to protect themselves."
How did you prepare yourself to play a character with this much emotional baggage?
"When I read the script, she just struck me as being incredibly selfish, so so selfish, completely unable to sympathise or empathise with other people. And I wanted to do more research on how someone could be like that and not be aware what they were doing was unfair, and I started researching about narcissistic personality disorder.
"These people, for various reasons, because of some trauma in childhood or whatever...they are the most important person in their own world. So, in an extreme situation you could see how that would affect and inform their decisions. So, that was where I started from, and obviously I've had the benefit of reading the whole script, so I could decide what would have happened when she was younger to have such an extreme effect on how she behaves in adult life."
From what you've said Biba seems like an extremely unlikeable character. Are you okay with playing someone like that?
"Yeah, they're so much more fun. I always want to play the bad guy. Well, she doesn't strike you as unlikeable. She's very bewitching and she's the kind of character, the kind of person, that you feel like no-one else in the world exists when they're speaking to you.
"They completely envelop you in this adoring gaze, and it's so wonderful, like basking in their presence. But when that's taken away, it feels so so dark and cold that you would do anything to get it back again.
"They give and they take and they give and they take to slowly get you under their ultimate control so they feel reassured. And you're not really aware that they're doing that because you just feel like you've done something wrong to upset them. It doesn't occur to you that they're just controlling you."
So she's more complex and manipulative than out-and-out unlikeable?
"Yeah, exactly. She's not just an out-and-out cow. You realise that there are things that are causing her to behave in a certain way, and you forgive her so much because she tries to make it up to you in her own way. So yeah, she's not just an out-and-out evil, bad character. It's more interesting than that."
Biba is described as a very glamorous character. Were there lots of big, extravagant costumes to try on for The Poison Tree?
Oh yes, there were so many... [they] were incredible. In Biba's room she hangs dresses on the wall like paintings. She adores the fabrics and the colours and the jewels. She surrounds herself with them all over the floor and they're like bedding - she has them across her bed .
"When she dresses up, it's like her armour. It's so much more than wanting to just look nice in a dress, it's vital to her character and how she's defined herself."
The book has a huge fanbase - are you worried about doing it justice?
"No, because the adaptation of it was reflected really well. It's always more difficult obviously because you don't have as much time. You can't include every single detail, but Erin also was pleased with the screenplays and pleased with the casting, so you know it's nice to have her blessing."
Did you read the book yourself?
"Oh yeah, it was impossible [to put down]. The phone would ring and I would silence it. I would be walking along the street reading it and bumping into people. It's a proper thriller. You get so invested in the characters so quickly and they're so vivid that you want to know what's going to happen. It's very well plotted out."
The Poison Tree is a bit of a change in direction for you - why did you decide to go from big Hollywood films to a relatively small British TV drama?
"Because whether it's in a studio movie in Los Angeles or whether it's an ITV drama, it's always the character that I respond to. It doesn't feel like a change of direction, it feels like it's another role, another character that I really liked and wanted to tell the story for."
It's also quite darker than some of the stuff you've been in recently. Was that a decision you made on purpose, to move into heavier territory?
"Yeah, absolutely. Like I say, it really is the role. If I read something and respond to the role, that's what happens, and if those happen to be a few comedies in a row, or not, so be it. But I was kind of hankering after a really dark drama to get my teeth into. I suppose I had an appetite for it."
The Poison Tree begins tonight (December 10) at 9pm on ITV1