The naked truth: Lizzy Caplan on 'Masters of Sex'

The actress, who made her film debut in hit comedy Mean Girls, stars in a new drama about the real-life pioneers of the science of human sexuality, Dr William Masters and Virginia Johnson.

They say Lizzy Caplan is 'adorkable'. It's a curious portmanteau that describes a new breed of acerbic women who have a passion for desserts and sarcasm; a Venn diagram of kooky, yet startlingly attractive brunettes that includes Zooey Deschanel and Katy Perry. Actor Caplan, now 31, made her film debut as an ostracised high-school goth in the hit comedy Mean Girls, later appearing as monster-bait in Cloverfield, and more recently as a sweary bridesmaid in Bachelorette. Fittingly, she has also starred in a number of short-lived, but cultish American TV shows, including Freaks and Geeks and Party Down.

"I have a special ability to spot TV shows that don't go past two seasons, that's my gift," she cracks, removing dark sunglasses. "No one seemed to care about Freaks and Party Down until after they were cancelled. I hope that luck doesn't continue."

Her new show, Masters of Sex, is a romp that she hopes will "run for years and years". In it, Caplan and Michael Sheen portray the real-life pioneers of the science of human sexuality, Dr William Masters and Virginia Johnson, whose research sparked an American sexual revolution. This 1950s drama retells a love affair ignited during unusual tests at a midwestern teaching hospital in St Louis, and it plays like an after-dark, uncensored Mad Men – certainly not safe for watching on the train.

"My dad hasn't seen it, nor do I think he will," says Caplan, wearily.

"I tried to get them to do a 'family edit', cutting out the stuff that my father wouldn't want to see, but they said it was too hard."

She cheerfully recounts a sex scene with Sheen, explaining how they were hooked up to a machine, totally naked. "He's a tender lover when he's got wires attached to his chest, but I can't speak for when he's truly unleashed!" She chuckles. "You know, I've done sex scenes before. But never under fluorescent lighting, and all these electrodes? It makes you long for the days of low-lighting and a bed." Caplan's huge eyes peer out from behind an oversized menu. "Instead, I'll just tell my dad the episodes he can watch. I can even give him the time stamps of the stuff that's OK. We'll figure it out. Have you got any almond milk?" A waiter has materialised.

We're sat in the Sunset Tower Hotel's Tower Bar, the infamous restaurant inside an art deco eyesore that is a monolith to Hollywood power and style. It is best known as a sanctuary of secrets to A-listers such as Johnny Depp, who practically lives within this paparazzi no-fly zone, this protected atrium where industry gossip – and today's sex-talk – is absorbed into its striped silk-and-walnut-panelled walls. Liz Taylor, Marilyn Monroe and Frank Sinatra were all regulars here, and John Wayne once lived upstairs in a penthouse apartment. Such was the actor's love for freshly-squeezed milk, he kept a cow on his balcony.

Left to right: Lizzy Caplan, Michael Sheen and Caitlin Fitzgerald in the forthcoming TV series, Masters of Sex (Craig Blankenhorn/Showtime)

"I did the whole non-dairy thing for a year," Caplan explains, almost apologetically, before impersonating one of those Hollywood stereotypes: "We're the only mammal that eats another animal's milk, it's like glue in your system," she whines. As one of those lesser-spotted natives of Los Angeles, Caplan says she has seen dietary fads come and go, and is largely immune to them. That gluten-free, low-carb, wheat-grass anxiety is at odds with her bullshit-free, low-maintenance charm. Her favourite perk since becoming an actress? A table-tennis table – and she's devastatingly good by all accounts. During a Q&A session with fans online, Caplan revealed she also plays 'Splosion Man on her Xbox, and that her favourite food is "hot dog nachos".

"None of my friends are in the entertainment industry," she insists. "They don't give a shit about Hollywood. I go to their homes, and you think I can get almond milk in their fridge? No!"

Lizzy Caplan grew up in a Reform Jewish family in the Miracle Mile, a well-heeled district four miles southf of our perch in the Sunset Tower Bar. She studied at the Alexander Hamilton High School's prestigious Academy of Music, before losing interest in the piano and pursuing drama. Her name is still embroidered on a soccer championship banner that hangs in the gymnasium. College was too much of a "time suck", she says, but there were enough high-jinks on the Toronto set of Mean Girls to rival any sophomore semester:

"Making it was ridiculous fun, all of us in a hotel. We were doing stuff that made each other laugh. I had no idea it would become an important cultural phenomenon. I saw Clueless five times in the theatre when I was growing up. Now I see new generations of kids are obsessed with Mean Girls. I love that."

Caplan wears: cropped trousers and blouse, both by Corey Lynn Calter, thisiscorey.com; shoes, by Greymer. Styling JAK at The Wall Group, hair Marcus Francis, make-up Sabrina Bedrani (Michael Muller)

Her next major screen role was in the sci-fi flick Cloverfield, a film notable for its motion sickness-inducing cinematography. "The budget was $20-25m… and for a monster movie that's nothing! What a cheap monster. Well, none of us got paid anything, it all went to that fucking monster!"

To give an impression of being filmed on handheld cameras, expensive cameras were bundled up and thrown down the stairs, she reveals, and in the film's tragic finale, Caplan's character perished along with the entire cast. Sipping her tall glass of black coffee as a cumulonimbus cloud of milk develops inside it, she ponders, as if for the first time, that the monster is the only surviving member of the cast. "I would watch a sequel if it's just about the monster, all alone in the world," she contemplates, "I would watch a film about a lonely monster."

Next, Caplan guest-starred on the hit vampire show True Blood as Amy Burley, the drug-addled girlfriend of Sookie Stackhouse's brother, Jason. In her debut sex scene, she rode Stackhouse topless, creating a television moment that has been immortalised by eager young men in dozens of thumbnail-sized online animations. These fanboy vignettes have already sprung up presenting scenes from Masters of Sex, and its pièce de résistance, the machine that monitors patients' heart rates as they experiment with sex.

"Michael and I observed so many actors doing it before we had to do it ourselves," reveals Caplan. "Our hats were off! You're surrounded by amazing crew and trained professionals, but you need to get past that discomfort. We couldn't even cover up between takes because of the fucking wires, and then you start to sweat, and all thef wires would fall off. You get used to enjoying small talk while completely naked."

Caplan says a sibling-like relationship with Sheen helped defuse any awkward tension. "Michael Sheen. The man. The legend," she sings, merrily. "He must be really popular in England, right?" Her eyes light up at the very thought of her workplace playmate. "It really could have gone in a number of terrible directions, but we became very close, very quickly. He was intimidating at first, but now there's lots of picking on one another, and we hold each other up. The schedule is brutal, so you spend a lot of time together. I like hanging out with Michael."

The sexologist Virginia Johnson wanted no part of the television adaptation of her life, so Caplan says researching the role was incredibly difficult. "It was hugely disappointing, and I held out hope when the show premiered that she would come and hang out on set, but that's no longer an option." Johnson died in July, before the show aired. "It's weird, I feel like I have this affinity to her, and when she passed away, it affected me pretty deeply. And I'd never met her. I have to ask Michael Sheen about David Frost."

Caplan says of Mean Girls: 'I had no idea it would become an important cultural phenomenon'

Sheen and Caplan were spotted partying together after Simon Pegg's premiere of The World's End, at LA's loosely British-themed pub The Cat and Fiddle. Despite a love for London thanks to a half-British boyfriend many years ago, Caplan remains appalled by Scotch eggs and Marmite ("it traumatised me"). Yet she modestly believes that if it wasn't for Brit director John Madden, who directed the show's pilot, she wouldn't have got the part in Masters: "If he'd have known more about American TV… Well, people have preconceived notions that I'm a comic actress," she says.

Funny or Die, that paragon of online comedy, has many goof-off Lizzy Caplan videos in stock, including one stand-out short in which she appears alongside her Cloverfield co-star TJ Miller, called Successful Alcoholics. She plays a terrific drunk, having learnt from Kirsten Dunst on Bachelorette how to spin around on the spot before shooting a scene. The 25-minute drama traces the downfall of two highly functioning twentysomething drinkers. "We wanted to shoot this dumb, funny thing and it turned into this very long, very sad thing. I am well versed in that world, not personally, but I know a lot about that world," she says carefully, suddenly watching every word.

Lizzie Caplan has dated Friends star Matthew Perry since 2006, yet details of their relationship are fiercely off-limits. Perry has been through rehab twice and at 43, he is now clean and sober. Today, the actor is a passionate advocate of drug courts where non-violent drug offenders can be rehabilitated through treatment centres instead of jail. In addition, he has transformed his sprawling Malibu mansion into a sober-living home for men, called Perry House, which opened earlier this year.

"For a lot of alcoholics and addicts, Successful Alcoholics affects them deeply," says Caplan. "One of the things they don't talk about in recovery is that when you do get sober, and you do finally deal with your shit, it's kinda boring! You need to figure out how to not stay in your house. I'm not an alcoholic, but someone said they should show that movie in rehabs."

Caplan will not even entertain any questions relating to her partner. When I mention his name, she cocks a hand to her ear and feigns deafness, until I change direction. She is proud of her anonymity: "I can stand right next to paparazzi," she boasts, "and I do that sometimes. I'm close with people who are far more famous, and they know how to avoid attention." Her private life is as protected as our table in this celebrity retreat, one that Jennifer Aniston calls her "little pocket of safety, a haven where you know you're not getting sold-out by the waiter". Here, the hotel owner Jeff Klein once hired a private detective to smoke out a room-service waiter who leaked Renée Zellweger gossip to a tabloid.

I'm behind enemy lines, but pushing my luck when I suggest that, should Caplan raise a family, there would be no hope of anonymity for her children. She lets out a deep sigh. "My children will be the children of an actress and that'll be a whole new set of issues." The wooden lollipop stick issued to stir her coffee has now been ripped into exactly 20 little pieces, and the almond milk in her coffee is curdling, fast. "Have you had the almond milk in the white bottle?" she asks, after a pregnant pause. "You get it at Gelson's. It's kinda new and beautifully packaged. It's not like this… it's creamy. Delicious. I'm obsessed."

Caplan explains that if her personal life became a tabloid drama, her audience would see all this real-world baggage on screen, lessening her dramatic effect, her believability. She argues that in the 1920s, when this tower was built upon the dirt track that is now Sunset Boulevard, female stars of the day would be known only for their screen presence. "In the Twenties, the screen sirens would never even have babies, as to do that meant they would be seen as human beings," she explains. "I read Masters and Johnson, and I've been forced to think carefully about it [the sexual revolution], and it breaks my heart for the women of America and what they went through, the blame they've shouldered, and it's not their fault."

'Masters of Sex' starts on Channel 4 this Tuesday at 9pm

Where are the other Mean Girls?

Lindsay Lohan (Cady Heron)

The former child star's fall from grace includes numerous stints in rehab and a spell in prison. The fact that a leggings line has been one of her bigger career successes of recent years speaks volumes about her subsequent film choices, which include the Razzie-winning, I Know Who Killed Me.

Rachel McAdams (Regina George)

Regina might have been "evil in human form", but since Mean Girls, McAdams has erred towards nice-girl-next-door roles in the likes of The Notebook and About Time, as well as picking up some critical clout by working with Woody Allen and Terrence Malick.

Amanda Seyfried (Karen Smith)

Style magazines might fawn over her and she's certainly an in-demand actress, but Seyfried is almost as dumb as Karen when it comes to picking roles; for every Les Misérables, there are half-a-dozen turkeys. See Red Riding Hood, Dear John and Lovelace for evidence.

Lacey Chabert (Gretchen Wieners)

Although she has worked steadily since, it's telling that the second role that Chabert is best known for is Claudia in Nineties drama, Party of Five. Gretchen's contribution of "That is so fetch" to the English language appears to be more significant than any-thing Chabert has done since.

Amy Poehler (Mrs George)

Poehler played Regina's cool mom (not a regular mom) and is currently starring in the award-winning Parks and Recreation, as well as gearing up to present the Golden Globes again with Tina Fey.

Tina Fey (Ms Norbury)

After writing and starring in Mean Girls, Fey left Saturday Night Live to star in a mixed bag of film comedies, as well as pub-lishing a best-selling memoir, Bossypants. Her greatest success is the glorious sitcom 30 Rock. Good for you, Glen Coco. You go, Glen Coco!

GILLIAN ORR