Why is Tamsin Greig so funny? Because she is. Indubitably. Her TV career spans two decades, via the surreal and hilarious hospital comedy Green Wing to the warm and witty Jewish sitcom Friday Night Dinner, by way of the so-funny-it’s-cringey comedy Episodes. Indeed, the night before we meet I saw her in her current West End comedy hit Jumpy. It begins with Greig walking on stage, dumping two big bags of shopping, opening a bottle of white wine and drinking a glass. That’s all she does, but the audience is in stitches. And continues laughing to the very end. “Every single night that happens,” she tells me. “I never thought it would get a laugh. But it does, every night.”

Is it your timing, or your face? I ask her. Maybe it’s just your face. I mean, you have a very droll face. This may not the best thing to tell a 46-year-old woman.

“Ha ha ha ha!” shouts Greig. “My funny face! As if… as if I’m on stage saying something serious and there’s this great big face gurning at them, making them laugh.”

Yes, but in a funny, warm way. You’re very warm, I add.

“Maybe people recognise that we all generally have that feeling inside of utter inadequacy,” says Greig, “and we all wear a mask that is hiding a sort of ‘help me’ feeling.”

Well, there’s no doubt we have all felt like dumping the shopping the minute we get into the house, and hitting the wine.

In Friday Night Dinner, which returns this week for its second series on Channel 4, Greig plays a rather formidable mother to two adult sons and wife to a hapless husband who never wears a shirt indoors. The joke is that, as a Jewish family, they all sit down to dinner together every Friday. It is chaos, with Greig trying in vain to carve the chicken while the men run riot around her.

Once again, she is very amusing, albeit in a brunette wig. I know she is in a wig because in Jumpy, where she plays a feminist turning 50, and in Episodes, in which she plays a British writer in Hollywood, her hair is short and streaked with grey.

As it is now we’re chatting. I suspect that the brunette is the wig, and the grey is the natural Tamsin. Although you never know. It could have been done artificially - after all, her character in Jumpy is an unreconstructed Greenham Common type, the sort who would have revelled in being streaked with grey.

Is your hair naturally like this? I ask.

At this point I think Tamsin Greig is going to fall off her chair. “Oh ha ha ha hooo haa,” she hoots, as if getting your hair artificially streaked with grey for a role playing a feminist is the funniest thing she has ever heard.

“I love that! HA HA HA! Is your hair actually naturally salt and pepper?” she asks, rhetorically. “How DO you get the grey just around the ears, that must take a LOT of time at the hairdresser’s! Ha ha ha.”

Maybe she hasn’t read the papers this morning. I feel the need to explain that we are discussing the Grey Issue because of Fiona Bruce saying she feels she must dye her hair, and Miriam O’Reilly saying she never will again, and even George Entwistle, new director-general of the Beeb, saying grey-haired women must all flock to be on screen in his brave new enlightened BBC.

Greig is reluctant to enter the debate. Having been in the headlines for her decision to resist the pressure to reach for the hair colour, she says: “I just felt… sort of amazed that it was newsworthy. When we were growing up, women in their late 40s generally didn’t dye their hair.”

We chat about feminism, and the paradox of why Matt LeBlanc, her co-star in Episodes, looks distinguished with grey hair but “women become more and more invisible.”

Greig, who is married to fellow actor Richard Leaf and has three young children, has evidently thought about it. Perhaps the fact that Episodes a British/American co-production, has done so well in the States and has been commissioned for a third series, might have something to do with it. Becoming a high-profile star in the US will mean that Greig has to grapple with the way female leads, even comic ones, are expected to do physical “maintenance”. Greig doesn’t do maintenance. She tells me she hardly ever exercises, bar the odd run in London’s Richmond Park, and when I mention Botox, she nearly falls over.

“Maybe this whole obsession about colouring our hair is about our inability to grow up. To let go of the fact we aren’t children any more, and the whole thing about changing our faces and looking young, and 60 being the new 40, is maybe we don’t want to let go of our childhood. Maybe that is where our angst is, that we are not allowed to be older and still have worth.”

We discuss the crazy fact that as role models, older women can be like Hillary Clinton or Hilary Devey - that is, running a country or running a giant industry - but if they are not like the two Hilarys, then they are completely invisible.

And yet with age Greig has become ever more visible. She remains a popular character, Debbie Aldridge, in The Archers on Radio 4, won an Olivier Award in 2007 for her role as Beatrice in Much Ado about Nothing in Stratford, and has had glowing reviews for her current play, not to mention success in two primetime sitcoms.

All that career success can’t have left much time to be a domestic goddess.

“What is wrong with being someone who stays at home and creates an amazing home?” asks Greig. She hopes that Friday Night Dinner will help to redress the balance, since she plays a middle-aged woman who delights in home and family. “She works, but she also welcomes her family home, back to a place where people want to come and have dinner on a Friday night, even though it’s chaotic. The Jewish culture holds that out as something to be praised, that has worth.”

And the humour? “The humour comes, from the fact her sons start running around the table and not sitting down and eating. That is where the comedy comes in. It was the same in Green Wing. We were supposed to be saving lives and Stephen Mangan was sitting playing a Bontempi organ.” She shakes her head. “That was an insane job. We were working in a real hospital, falling over and singing loudly, having sex in cupboards, while people were being trollied past us who were about to die.”

And as she talks, her beautifully high cheek bones, high eyebrows and innocent doe-brown eyes sort of combine with what she is saying and I nearly fall off my chair giggling. 

Friday Night Dinner returns tonight at 10:05pm on Channel 4