Turns out it's not just Dexter keeping secrets. Jennifer Carpenter, the actress who plays the serial killer's foster sister Debra, has harboured a few of her own since starting on the hit show in 2006.
"I had a handful of decisions I'd made and didn't share with anyone," Carpenter says, on the phone, while on hiatus from the show.
"Things about back story, justifications for how she moves and why she talks the way she talks. I filled in a lot of blanks for her."
Before the pilot was filmed, she imagined Debra had feelings for her brother, or rather "that she was charmed by him". It's the only secret she will share because, as fans will know, that's how things played out when Deb told him she felt more than sibling affection for him at the end of season six. Though that storyline may not develop any time soon, given Dexter's attraction to fellow killer Hannah, the back story has served as a tool to help Carpenter deal with their biggest story to date. Deb now knows Dexter isn't just a blood splatter analyst but a sociopath who fulfils his bloodlust by dispatching of bad guys after dark. At the end of season six Deb caught him in church, stabbing the Doomsday Killer in the chest. Since then, the newly appointed Miami Metro Homicide Lieutenant has struggled to come to terms with her duplicitous brother's behaviour. Complying with his murderous deeds is "more about honouring justice than honouring his compulsion", Carpenter explains. And though she does indulge in some shady behaviour this season, there is a line she won't cross.
"I said to [executive producer] Scott Buck that I want what we told the audience to remain true. I don't want to lie to anyone. Because I really did not think it would be fair to make it a Bonnie and Clyde situation. Besides, we've seen that with Julia Stiles' character [Lumen] and we saw it with Miguel Prado [played by Jimmy Smits], and Deb is neither. It really is a moment to moment thing."
The big revelation is part of Dexter's falling house of cards, as the writers work towards the show's end next season. So far, any concerns that Dexter would turn to farce or jump the shark - something akin to Mulder and Scully getting together in The X Files - have been alleviated. Debra's exploration of Dexter's dark side has turned season seven into one of the most riveting and complex. It's renewed the energy on set, says Carpenter.
"The show has always been complicated, it's the nature of it. It's hard to navigate but now in a way because I'm aware, and I've seen the face of Dexter's Dark Passenger, it inflates the scene."
Though it terrified her to play those scenes, worrying it would compromise Deb's values, Carpenter says she always knew that one day Debra would find out the truth. But she did have creative input into episode five, requesting a scene in which Debra repeatedly drops the f-bomb, and other related profanities, in the lift.
"I was worried that Debra was going to appear like a machine, like there's nothing she can't handle, like she's unbreakable. She's always a little fragile. I wanted the audience to have a moment of privacy with her, to see her struggle and see how much she's actually holding back in a scene with other people."
Holding back is something Carpenter does too. She didn't expect Dexter would even make it past the pilot stage, finding it hard to believe the network would run with a show about a serial killer. Since being thrust into the limelight, she's been determined to keep her private life private. Her PR team have requested no personal questions during this interview, which basically translates to mean no questions about her relationship with ex-husband Michael C Hall, something of an occupational hazard considering he is the actor she shares most scenes with. She is wary when questioned about their working relationship, but will say they support and respect one another. But she's happy to compare her personality to Debra's.
"We are alike in that we are resilient and engaged in life. But I tend to find the joy in things in a way that she can't, understandably so."
It's a role that has seeped into her daily life. Even Debra's foul mouth has been an influence.
"The first year I got in the habit of it and I went home for Thanksgiving and my father pointed it out and I realised how unattractive it was, and it wasn't really a reflection of who I am. So I made an effort to curb it. Not to say I've retired it. I just don't always pull it out of my back pocket."
If the cursing is too much to handle, or the bloody crime scenes turn you green, don't expect Carpenter to offer much sympathy.
"Television is very powerful and if you do not feel that you can stomach it or trust yourself with it, then you should turn it off. Dexter is not a how-to by any means. I think it is an accelerated slice of life that hopefully by seeing enough will keep you in line. It's meant to be entertainment and if people take offence to that then that's their issue and not our writers'."
Not surprisingly she has mixed emotions about her gig coming to an end. But for all her love for the character, it's also given her perspective.
"It's funny, I feel like Debra's life is so complicated and fraught with impending doom and disaster every other minute, that I have the luxury of really appreciating my life."
After training at the Juilliard School in New York, Carpenter made her Broadway debut in Arthur Miller's The Crucible, starring Liam Neeson and Laura Linney. Her breakthrough film role was playing the titular character in The Exorcism of Emily Rose in 2005. This year she shot the romantic comedy Ex-Girlfriends. But TV is her preferred medium.
"I imagine it doesn't happen this way [on Dexter] very often. Pretty much all of us started in theatre, so the work ethic is there. We're all honoured to have a job and we are all on the same team, working to win every day. The show is extraordinarily satisfying because it is so incredibly challenging on every single front - personally, emotionally, even just approaching the work. Sometimes I come at it cowering because it is the most difficult and simultaneously rewarding chapter of my life thus far."
How she'll live up to that, she's still not sure.
"My dad worked in a factory for 30 years so there's something in my personality or DNA that really appreciates showing up at a job every day. And I really love the slow burn of television story-telling. I'd be really interested in doing another series. But I may have to go on a yoga retreat for seven months first. I don't know. We'll see what happens."
When: Wednesdays, 8.30pm repeated Saturdays 9.30pm