After his "mong" controversy on Twitter, about the whether or not the word is still used pejoratively against people with Down's syndrome, and his recent spat with American fundamentalists when he posed as Christ crucified on the cover of New Humanist magazine, Ricky Gervais might seem set on yet another collision course with his new Channel 4 comedy, Derek. Gervais plays the eponymous Derek Noakes, a kindly but simple helper at a nursing home, who appears to have learning difficulties. Perhaps understandably, Gervais isn't in a diagnostic mood and doesn't specify Derek's condition – if "condition" is indeed what it is.

"I've never thought of him as disabled," is how Gervais puts it more bluntly when we catch up after a screening of Derek at Bafta (quite a showcase for what is essentially a sitcom pilot). "He's not that bright but he's cleverer than Baldrick, he's clever than Father Dougal (Ardal O'Hanlon's character in Father Ted)... he hasn't got as big a problem as Mr Bean.

"When I portray a disabled person I get a disabled person to play them – the woman who was wheelchair-bound in The Office, the guy in Extras with Down's syndrome was a Down's syndrome actor. If it was autism I'd get Dustin Hoffman..."

Gervais started developing Derek for his stand-up act more than 12 years ago – long before The Office catapulted him to stardom and, it sometimes seems, ceaseless controversy.

"Originally the character was an excuse to see the world differently," he says. "He lived with his mum but I thought that was quite restricting for a sitcom – and the change to the old people's home was after I watched The Secret Millionaire (the Channel 4 reality show about anonymous philanthropists) – it was always these people with huge problems who were helping other people. I thought about having Derek help old people because no one cares about old people in this country... I think it's perfect for now." Indeed, Derek might shock some viewers, albeit not for the expected reasons. What's surprising is that it's a remarkably tender, even slightly mawkish at times, sitcom, in which the title character is an innocent who loves Rolf Harris, Jesus ("because he's nice"), Britain's Got Talent, and clipping the toenails of the elderly residents – the tone flowing between nuanced comedy and poignancy.

"I think real life is like that," says Gervais. "It's a comedy drama. But then everything that's real and is done with a bit of compassion is a comedy drama – you try and have a laugh all the time and then you find a lump...

"I do like getting close to real emotions, I've never been scared of that. But people assume my work is outrageous and cynical, and it's never been. I mean The Office was never cynical – there was always happy endings in The Office and Extras – and this is all about kindness and forgotten people on the peripheral of society. It's not a class thing, it's not an education thing... I live Hampstead and there are people there who never wear socks."

Refreshingly, there are no celebrity cameos, as there were in Extras and Life is Short, in Derek. "I think it's time to have an antidote to fame – I just want to do normal people again," says Gervais, who nevertheless persuaded his old friend and acolyte, Karl Pilkington, fresh from his success as a kind of anti-Alan Whicker in Sky1's An Idiot Abroad, to don a comedy wig and make his acting debut, playing caretaker Dougie (a bemused northerner, so no great stretch there). "I knew if he did what I said he'd be great," says Gervais. "I don't mean that arrogantly. It's written for him and some of the things I've written for him to say he could well say, although you can never predict him."

Gervais illustrates the point by telling how, only this week, he has persuaded Pilkington to sign a contract for The Short Way Round, a tour with another of his collaborators, Warwick Davis from Life's Too Short. "Karl cycles round the world with a dwarf in the basket... when I told him that idea he didn't say, 'that's ridiculous,' he said, 'so I'm doing all the pedalling then?'"

Does he expect to be attacked for his portrayal of someone who appears to have learning difficulties? "Yes, I'd be confused if they didn't," says Gervais. "Every week it's been the end of my career, for the last 20 years... I mean I started with a backlash. What can you do? If you start making it for reviews or rewards or the public, you don't get anywhere. I don't make it for the public, I make it for me."

As for his Twitter spats – Gervais said that the so-called "mong-gate" was blown out of all proportion. "That's gone into myth and legend," is how he puts it. "I said at the time I never used that word to mean Down's syndrome – never would, never have – and then when I was contacted by the Down's Syndrome Society and they said it's still used, I went, 'okay, I won't use it again'." An atheist since boyhood, Gervais could still mend a few bridges with his Christian fundamentalist critics with his next project, although I wouldn't bet on it.

"It's about an atheist who dies and goes to heaven and he's not so smug," he says. "I play God... it's a stretch." And then there's the continued preparation for the film of his Flanimals children's books ("it's taking forever"), as well as European and American, Michael McIntyre-sized arena tours for his stand-up act.

"And we've just started writing a one-hour special for Life's Too Short, which at the moment is turning into a spin-off series with Les Dennis, Barry off EastEnders and Cheggers [Keith Chegwin]... we couldn't resist it."

"We", it almost goes without saying, are Gervais and his regular writing partner, Stephen Merchant, and it's striking that he wrote Derek without Merchant, as well as directing and starring in it. "It's fun collaborating but it's still a compromise," he says.

"Two people is still a compromise. That's why I like live [comedy].... you can say anything and there's no interface between thinking it and saying it – it's more liberating." Does he worry that he might lose objectivity with so much creative control?

"I don't care. I remember Christopher Guest said to me once: 'How will we know if we've completely lost it?' And I went: 'Who cares? It'll be great... it'll be fun.' And it's not letting somebody else down. I don't mind letting myself down. If I'm in charge I don't care what happens because I get my own way – if you get your own way it's how you wanted it at least."

In the event Derek is an improvement on Life is Short and might be his best work since Extras. He certainly believes there is scope for a full series – in fact Gervais is already busy writing it. "It's certainly got legs because there's lots of stories to explore," he says. "It's essentially an ensemble piece like The Office and Extras. It still adheres to the sitcom format – it's a family against the world, which all the best sitcoms are."

He goes on to describe how Derek is trapped in this nursing home in the same way that Harold Steptoe is trapped by his father, and Ronnie Barker's character is literally incarcerated in Porridge. "It's already quite a melancholy place, an old people's home." Ah, yes, the old people – will his portrayal of the elderly provide another stick for his detractors?

"They're not quirky as in, 'don't old people say the funniest thing,'" he says. "They're from all different walks of life and have real fears and real issues. They're not props, they're not a backdrop – I really want to develop them as real characters."

Indeed, after a couple of opening gags, in which two residents of the nursing home are depicted as deaf and senile, that had you fearing the worst, Gervais opens up a rather moving storyline involved an old lady whom Derek treats as a mother substitute, particularly liking it when she strokes his hair.

"The difference with this and other sitcoms, and certainly other sitcoms that I've done, is that there's no real vein of irony," he says.

"People are saying exactly what they mean. Derek's certainly not delusional like David Brent, and there's nothing more fun than playing a delusional character. There are still three David Brents in every series of The Apprentice." Yes, but while there is obviously a crossover between Brent and Gervais – not least the compulsion to perform – how much of Gervais is there in Derek?

"Every character I've played has got lots of me," he says. "And it's not just me. Gareth [in The Office] is based on someone I went to school with, that's why he's like a 14-year-old kid... Cemetery Junction, that's my childhood. If you don't [do it] you're just guessing."

Indeed, Gervais gives one of his least overtly comic performances to date – something of a riposte to those who think that he is not a versatile performer. "Some people confuse what versatile is," he says. "Dustin Hoffman does the same thing, he just does it brilliantly. Woody Allen, Laurel and Hardy..."

Would he like to do more straight acting? "It's difficult for a comedian to be taken totally serious. My baptism of fire was The Office where I'd never acted before but I felt comfortable because I knew the character so well and I knew what I was emulating – it's a documentary. What's more daunting to me is if somebody said, 'do this Shakespeare play... there's a way to do it, the audience know when you go wrong'. That would be daunting to me.

"I've often said to directors who've cast me, 'you can find people who are better than me', and they look at me as if I was mad. Actually I think I can play demented and weird – I'd like to play a psychotic serial killer like Dexter. One Flew over the Cuckoo's Nest would be the ideal role."

It's funny he should say that because One Flew over the Cuckoo's Nest had come into my mind while watching Derek. Was his character based on anyone in particular?

"I better be careful about what I say, but Derek is sort of based on some autograph-hunters I've met around the country. I didn't know the satchel was still in [fashion]..."

And as I exit Bafta there are three overweight middle-aged men waiting outside who might well be autograph-hunters, although what look like satchels round their necks turned out to be cameras. They are the autograph-hunter's mercenary, more aggressive. metropolitan cousins – the paparazzi.

'Derek' shows on Channel 4 on 12 April at 10pm

From extras to stars: How Ricky makes careers

Martin Freeman

Freeman was doing the usual rounds of 'Casualty' and 'The Bill' until his portrayal of Tim, the lovelorn, prank-playing worker in 'The Office' in 2001. After roles in 'Love Actually', 'Hot Fuzz' and as Arthur Dent in the movie of 'The Hitchhikers' Guide to the Galaxy', his star status was confirmed BBC1's 'Sherlock', in which he plays Dr Watson. See him next as Bilbo Baggins in Peter Jackson's 'The Hobbit'.

Ashley Jensen

Scottish actress Jensen's career had been steady but unspectacular for 15 years when she co-starred in 'Extras', playing fellow supporting artist Maggie. Since 'Extras' finished in 2005, Jensen has played best friend Christina in three seasons of 'Ugly Betty', and co-stared in another US sitcom, 'Accidentally on Purpose'. Hear her next alongside Hugh Grant as one of the voices in the Aardman animation, 'The Pirates! Band of Misfits'.

Lucy Davis

Comedian Jasper Carrott's daughter, Lucy Davis, was finding it hard to graduate from guest parts in shows like 'Holby City', 'Doctors' and 'The Bill'. Since 'The Office', in which she played receptionist Dawn, and a starring role in 'Shaun of the Dead', Davis still does regular guest spots, but this time in more rewarding US shows such as 'Ugly Betty', 'Californication' and 'The Mentalist'. See her next headlining the British thriller 'Nitrate'.

Mackenzie Crook

The former stand-up comedian has turned into a surprisingly versatile actor since his star-making portrayal of the nerdish Gareth in 'The Office' , from the Royal Court production of 'Jerusalem' and a sadistic soldier in BBC1's 'The Accused' to Russell Hardy in the Ian Dury biopic 'Sex & Drugs & Rock & Roll' and, of course, one-eyed sailor Ragetti the 'Pirates of the Caribbean' blockbusters. See him next in the British rom-com 'Cheerful Weather for a Wedding'.

Kerry Godliman