Baring all is par for the course for many actors, but Ashley Jensen's latest on-set striptease was a fairly unusual one.

"I'm the only person in the world who has stripped off in front of Colin Farrell and been laughed at," reveals the Annan-born actress, who worked with the Irish actor on dystopian film The Lobster, which comes out next year.

"Then he said I'm probably the only person who has watched him strip off and laughed."

Full of self-deprecating wit and cheerful chat, it's no wonder the Scottish actress has been the go-to woman for screen best friends, famously in Ugly Betty as Betty's office ally Christina, and in Extras, as socially inept but big-hearted struggling actress Maggie.

But after years of dazzling on the sidelines, the 45-year-old is stepping out as the leading lady in Sky1 Christmas crime caper Agatha Raisin And The Quiche Of Death, which will air on Boxing Day.

In the one-off drama, based on the bestseller by M.C. Beaton, she plays high-powered publicist Agatha, who swaps life in the city for the tranquillity of the Cotswolds. Peace is short-lived though, and she finds herself embroiled in a murder mystery triggered by the quiche she makes to win over the villagers.

"Everyone points the finger at Agatha, because it's her spinach quiche that's the weapon," explains Jensen, who stars alongside Robert Bathurst and Hermione Norris. "And so in order to clear her name, she decides to work out who the killer is."

The opportunity to front a show, especially one with a "slightly ridiculous" sounding title, was too good to pass over, she admits.

"It's the first time I've done something like that, where it's me at the helm of it, and I enjoyed it," says the mother-of-one; she has a five-year-old son, Francis, with her husband, Human Traffic actor Terence Beesley.

"I think most actors quite like something they can be in where they have a beginning, middle and an end. I've played a lot of characters who have been the best friend, so it was nice to be a person, a woman in her 40s, leading a series.

"There's not that many of them, really. There are a lot of really male influenced things out there."

Another selling point of the script was the idea of playing a fish out of water. Jensen, who spent a couple of years in Hollywood, is well versed on this.

To begin with, there were the bursts of applause when she announced her name at the Ugly Betty read-throughs to adjust to.

"I'd say, 'I'm Ashley Jensen, I'm playing Christina', and everyone would go, 'Yay!' and clap, and I'd be like, 'Brilliant, I've just said my name and got a round of applause'."

Then, as 'the voice' of Channel 4's warts and all factual series Embarrassing Bodies, many more things were lost in translation, as she recorded her narration in an American studio.

"I used to go, 'Is it vag-eye-nal or vag-in-al, I can't remember? Which one is British again? Oestrogen or estrogen?'" she recalls, laughing. "And the producers were going, 'Don't go all American on us!'"

Far from "going American", the move to the US sharpened her sense of cultural identity.

"You're aware of your own culture as a British-Scottish person, and you're aware of the differences between American and British," says the actress, who thinks it'd be brilliant if there was an Ugly Betty film, meaning a chance for a "wee jolly out in LA".

"You take the good things from there, and the good things from here, and learn from it all a bit."

Now firmly rooted in the Bath countryside with a base of new pals, largely thanks to her son ("It's a wee bit easier when you have a child because you can make friends that way"), she is pleased to be away from some of the less enjoyable aspects of Los Angeles life.

"I don't miss the veneer of it," she says, though she does particularly miss her Ugly Betty co-star Ana Ortiz, who played Betty's big sister and was expecting her first child around the same time as Jensen.

"I found all that quite wearing, knowing everyone was aspiring to be something rather than being present, do you know what I mean?

"Rather than saying, 'That's a nice cup of coffee, I like your necklace', everything was a little, 'I'm gonna be, I'm gonna be, and I want to be, I want to be'."

And much like her latest character Agatha, she found herself craving more of the good life.

"When I went to America, I came back [to the UK] and I felt like I wanted to personally simplify my life a bit," she says. "I think people get to a point in their lives, where they want to do that a little bit. I don't want stylists and publicists and somebody who blows leaves in the garden for me.

"I felt awfully uncomfortable with that sort of life."

So although she met a lot of "lovely people" out there, she is pleased to be home.

"I played at Hollywood, and I'm really glad I did it," says Jensen.

"I enjoyed it, and I absolutely went for it and had everything I could - the manicures and the publicists... But there came a point where I just wanted to hide up a hill and pretend Twitter wasn't happening.

"I'm very happy where I am now."

EXTRA TIME - FIVE MEMORABLE TV DEATHS

:: Kenny McCormick, South Park - Syphilis, suicide or auto-erotic asphyxiation; countless causes were the death of poor Kenny McCormick, in foul-mouthed cartoon series South Park.

:: Matthew Crawley, Downton Abbey - Still reeling from the tragic death of Sybil, halfway through series three, viewers cried into their mince pies when Matthew Crawley (Dan Stevens) was killed off on Christmas Day.

:: Hayley Cropper, Coronation Street - Hankies were at the ready when poor old Hayley, played by Julie Hesmondhalgh, ended her life after suffering with terminal pancreatic cancer. Despite being one of the most controversial storylines in the soap's history, it was hailed a success by critics and watched by over 10m viewers.

:: Ned Stark, Game of Thrones - There were shock waves when Sean Bean's Ned Stark was brutally executed in the first series of this fantasy epic. Falsely confessing to conspiracy in a bid to save his beloved daughters' lives, the merciless King Joffrey chopped off Stark's head and paraded it on a spike.

:: Sherlock Holmes, Sherlock - Fans spent two years speculating whether the quick-thinking detective was still alive, after he dived off St Bart's Hospital at the end of series two. But being the clever sausage he is, the fall didn't really kill him, and he was back to explain away his brush with death in the third series.

:: Agatha Raisin And The Quiche Of Death is on Sky1 on Friday, December 26