Mackenzie Crook is in his shed. Behind him is a model ship, a plant pot crammed with cacti, a tin mug full of pencils, a fish tank and a small fishing net. “Do you remember Pokemon Go?” says the Office and Pirates of the Caribbean star, scratching a tattooed arm. “You’d have to go out and find these monsters on your phone, and if you wanted to see the rare ones you had to go further afield. That just blew my mind. Why don’t they get a pair of binoculars and go out and watch birds? Because that’s exactly the same, and they’re real.” He sighs. “I shouldn’t say this out loud. I’m gonna sound like an old man now.”

Crook is a gentle soul. Softly spoken, with a touch of melancholy about him, the 50-year-old is dressed like a scruffy trucker – checked shirt, fraying cap, his frame just as wiry as ever. He has the same self-effacing air as Andy, who roams the English countryside in the bucolic BBC comedy Detectorists, scanning for treasure and mostly finding ring pulls and pound coins. And he loves nature just as much as Worzel Gummidge, the benevolent scarecrow who returns for two Christmas specials this week. In fact, he’s the antithesis of the character that made him. Thank God.

“He’s a bit of a monster actually, Gareth,” says Crook of his breakout role in The Office. He just rewatched the game-changing mockumentary series for the first time since it aired 20 years ago, and seems to have had mixed feelings. “I didn’t remember him being quite so… urgh. The things he comes out with when he’s trying to chat up girls are diabolical.” He may be “urgh”, but he is also brilliantly, singularly funny. The weedy assistant (to the) regional manager of Wernham-Hogg, with a nightmarish pudding-bowl haircut and an ill-fitting suit, Gareth is the perfect henchman-slash-wingman to Ricky Gervais’s detestable boss David Brent. He is obsequious and humourless. Despite looking like he’d lose a fight with a piece of paper, he claims to be a former member of the territorial army.

“It’s an odd performance,” says Crook, brow furrowed. “I seem to be doing much more than everyone else. Everyone else seems to be very natural, and then I just seem to be coming in from a different angle. I’m more cartoonish than any other character.”

He is doing himself a disservice, of course – he even manages to wring some pathos out of Gareth, like when he learns that David Brent is leaving and breaks down crying – but he concedes that the rest of the show is pretty great. “Although a lot of it was based on cruel humour, the best bits are the bits that have so much heart and bring a lump to your throat,” he says. “Without those bits, it would have been just a bit relentlessly cruel.”

The show transformed comedy, which back then was full of glossy stars and canned laughter, and its influence has only become more apparent in the years since it ended in 2003. But seen through contemporary eyes, the extent of the ironic racism, misogyny and ableism is quite shocking. Ricky Gervais recently claimed that the show would be “cancelled” – in the modern use of the term – if it aired now, though he later said he was joking.

“I suppose it’s a document of its time,” says Crook. “What we were portraying back then were things that were being said in an office environment, and people were getting away with it. So we were pointing it out – laughing at those people who were behaving so badly. But yeah, I don’t think those things could be said in a workplace anymore without somebody picking up on them. So no, you wouldn’t have it now in a comedy.” People still enjoy it, though, he adds. So no need for “cancellation” then? “It was cringey at points but no, it didn’t need to be wiped out.”

He’s a bit of a monster actually, Gareth Keenan

When The Office ended, Crook was glad, in a way, to leave Gareth behind. Not least because it meant saying goodbye to the haircut, which he was reluctantly sporting when he married his wife Lindsay, and because he was growing tired of “going into a pub and young blokes getting me to speak to their brother on the phone… that would spoil a night out”. But also because it meant taking on new, less diabolical roles. Where Gervais has become a crueller public persona over the past decade, Crook has gone the opposite way. He had a brief dalliance with cinema – he played the one-eyed rapscallion Ragetti in Pirates of the Caribbean, a role he loved but involved far too much waiting around out at sea, and a delivery man in the reviled Sex Lives of the Potato Men, which he describes today as “an awful film” – and then decided that cinema wasn’t for him.

He retreated to the world of TV, where there was less waiting around and more room to create. First he made Detectorists, a low-key delight about two metal detectors – sorry, metal detectorists – that he wrote, directed, and starred in with a curmudgeonly Toby Jones. Then came Worzel Gummidge.

If resurrecting the sentient scarecrow from the creepy Seventies children’s show seems like an odd decision, it made sense to Crook. “Early on in my career, people were always suggesting parts that I should play: there was always Stig of the Dump, Catweazle… and Worzel Gummidge.” So he decided to give it a go. “For me, it feels like an evolution from Detectorists, which was very much planted in the English landscape, and where we hinted at myth and magic, but with Worzel Gummidge I can make that upfront, and make it into a magical story for kids.”

It wasn’t easy. “One of my major worries was whether we’d got his look right. I didn’t want him to be creepy-looking. I wanted him to be alarming at first glance but then once he starts smiling and speaking, you realise he’s a nice guy. But, of course, still photographs were released first, and I was really nervous that people would just look at that and go: ‘Oh, this is gonna be terrifying.’” His fears aren’t completely unfounded. I was resistant to Worzel’s charms when it first aired in 2019, mostly because of the disturbing growths sprouting from his chin – but the show is wonderful. Part parable about protecting the environment, part cosy, Christmastime comedy, it’s the story of two city foster kids who move to a farm and make friends with a bumbling scarecrow. Crook is a joy – as silly and naive as a bunch of twigs in a Napoleonic red coat should be – but that undercurrent of existential angst is there, too.

“First and foremost, I wanted it to be funny for kids,” says Crook when I suggest that there are analogies at play. “And second to that, I suppose, was an appreciation of the natural world.” Crook once considered buying a sports car before plumping for eight acres of protected woodland instead. “I didn’t want to hammer home a message about being environmentally friendly because kids are so on board with that already. It’s the older generation that needs to catch up.”

Funny and uplifting though it is, there is also a sadness humming away in the background of Worzel Gummidge. In one of the new episodes, in which Worzel visits a funfair in the dead of night to avoid being spotted by humans, he tells the kids: “I know Earthy says we should all stop wishing we was humans, but I can’t help it sometimes. You don’t know how lucky you are.” Though it is mostly full of whimsy, there are times when the show forces you to think about how truly depressing a life confined to one place would be. “It’s funny,” says Crook, “people point out things to me in my writing and I think, ‘God yeah, I suppose I did mean that, but I hadn’t really thought that deeply about it.’ I mean, part of his job is to impersonate a human, but without communicating with them. So he has to pick up clues about what it’s all about to be a human. From his perspective, humans have all this stuff at their disposal, and they could be really good.”

It must take a lot of confidence to write, direct and star in your own show – and yet I keep reading about how timid Crook is. In 2007, he starred in a stage production of The Seagull alongside a then-unknown Carey Mulligan. “In rehearsals,” the director Ian Rickson would later recall, “Mackenzie and Carey would sometimes be so shy that they’d ask me to look away while they were acting.” He’s on stage again next year – in the West End, no less – reprising his role alongside Mark Rylance in the acclaimed Jerusalem, 13 years after it premiered. But what about that shyness?

“Am I shy?” asks Crook. “I suppose I’m shy.” He thinks for a moment. “I don’t know if it’s shyness or social awkwardness. I get really anxious about meeting people, forgetting people’s names, knowing whether I’ve met someone before…” Luckily, all of that fear disappears when he’s acting. “Performing is a release from being awkward. As soon as I’m on camera with a turnip head in the middle of a field pretending to tap dance, I’m not socially awkward.” He laughs. “It’s actually quite a release.”

The ‘Worzel Gummidge’ Christmas specials air on BBC One on Tuesday 28 and Wednesday 29 December