NOT SO SERIOUS: "People don't know that I can do funny," says former X-Files star Gillian Anderson.

Playing the boss of a government intelligence agency might be considered a step up for American actress Gillian Anderson.

After all, she spent nine years protecting the population from supernatural terrors and exposing sinister conspiracies as FBI agent Dana Scully in cult 1990s television series The X-Files, but never got a single promotion.

She never even got her own office.

In her new film, Johnny English Reborn, the sequel to the 2003 spy spoof starring comedian Rowan Atkinson as the bungling titular agent, she is the head of Britain's MI7 and has a top-floor office suite. Anderson, however, feels far from in charge.

"The idea of me being the boss of anybody just really cracks me up," she says.

"I also just did a film with Clive Owen called Shadow Dancer, where I played his boss in MI5.

"I don't know why I keep being asked to play bosses," she ponders.

"I might need to see a therapist about that. I don't feel very authoritative in my world."

The 43-year-old's world is certainly light years away from the deadly serious characters she has carved a career out of - from the cool and clinical Scully to the grave Lady Dedlock in the BBC's critically-acclaimed adaptation of Charles Dickens' Bleak House.

By contrast, Anderson is easygoing and playful. She laughs a lot and, perhaps as a result of shouldering such heavy characters on camera, looks younger in real life.

She is a blonde, not a redhead, despite Scully's signature bob, and she alternates between an American and a posh British accent thanks to her cross-continental upbringing (born in Chicago, but raised in London until the age of 11, where she now resides with her three children, daughter Piper, 16, from her first marriage to X-Files assistant art director Clyde Klotz, and sons Oscar, four, and Felix, two, with partner Mark Griffiths, a vehicle-towing company director).

In Johnny English Reborn, Anderson is the straight man to Atkinson's rubber-faced antics.

She plays his boss, Pamela "Pegasus" Thornton, who is keen to protect the spy agency's fragile public image after English is recalled from exile to foil an international assassination plot.

Atkinson may take his comedy very seriously behind the scenes, but Anderson says the Mr Bean star had her giggling before they were even introduced.

"The very first time I encountered him, he didn't actually know that I was encountering him, because I was behind him while he was trying to open a screen door," she says.

"He was the only person in the room, so he wasn't performing for anybody, but just the way that he was tackling the door was extremely funny. I actually left the room and re-entered."

Johnny English is not her first foray into comedy. She appeared in 2008's How to Lose Friends and Alienate People and the 2009 art world satire Boogie Woogie.

"Part of it is that people don't know that I can do funny, so I don't usually get comedy coming my way.

"Some people don't know or don't remember that some of The X-Files were comedic episodes. David [Duchovny, who played her partner Fox Mulder] and I got to do a little bit of that."

Her favourite episode is the fifth season's hilarious Bad Blood, in which Mulder and Scully present wildly differing versions of an ill-fated vampire investigation.

It was a clever spoof on their relationship and the show, which pushed the boundaries of extreme possibilities, in general.

Few TV programmes have left as big a cultural imprint as The X-Files. Anderson had no inkling of the level of devotion until she stepped on to a shopping centre stage in Australia in 1996.

More than 10,000 screaming fans had crammed into the venue to catch a glimpse of the petite star during a promotional tour. People fainted.

Others hung dangerously over balcony railings and 100 police and security officers struggled to prevent a mass crush.

The scene was repeated throughout the country.

She says her recent visit Down Under for the Johnny English Reborn premiere was a less terrifying experience.

"People don't give a darn who I am now," she laughs.

"That [1996] was a very unique situation and I certainly wasn't expecting it."

Scully, the medical doctor turned FBI agent, made science sexy. She blazed a trail for strong female TV characters, a legacy that continues today.

Yet Anderson had to fight to get equal pay with Duchovny well into the show's run. She also had to overcome archaic directives from network brass in the early days - Scully was only allowed to walk behind Mulder in scenes.

The duo's electric screen chemistry, however, never faltered, even if their off-screen relationship buckled under the pressure of a primetime TV show's relentless production schedule.

Today, they say they are much better friends.

At a recent X-Files charity fan event in Los Angeles, Anderson cheekily asked Duchovny why she hadn't been offered a guest role on his new show, Californication.

"We can't afford you," he quipped.

"I'd do it for free," she said.

When The X-Files ended in 2002, an exhausted Anderson was not interested in Hollywood and relocated to England, where she reinvented herself as a proper British actress doing theatre and period dramas.

Her credits to date include The House of Mirth, Moby Dick, The Crimson Petal and the White and next year's Great Expectations, in which she plays Dickens' famously jilted bride, Miss Havesham.

"They are hard work, but there's something really magical about being part of a big costume piece," she says.

"Especially something that is based on a literary masterpiece, because there's a weight to it, an expectation - no pun intended - of wanting to do right by the beauty of the material and the depth of the story."

There is another reason Anderson likes self-contained period pieces. There is no chance of another nine-year stretch in one role.

Aside from the odd independent film and the second X-Files movie, I Want To Believe, in 2008, she has been content to stick to smaller parts, including an appearance in 2006's The Last King of Scotland.

While she would make an exception for a third X-Files film (the cast have all indicated they would return, but nothing has been made official), she is in no rush to become a leading lady again.

"Every time I do something that takes a long time, I am aware of how much it is taking away from my experience with my kids.

"I do try to keep the amount of weeks that I'm filming down. Sometimes you can block shoot things, so if a film is going on for two months, you can block shoot a character within three to four weeks.

"That's harder to do if you're a more central character, so it has to be something pretty great to make that decision to be on set for three months."

Johnny English Reborn is in cinemas now.