Self-doubt pushes 
me on, I’ll admit to that. Can I do it? Am I capable? But I won’t be happy, personally, until I give 
it a try.

And there’s a word which rather nicely sums her up. It is “mercurial”. She can be very earnest, and very deep one moment, the next

Smith has recently celebrated her 31st birthday and is just about to open at the Old Vic Theatre in London in one of the most complex roles in the female acting repertoire. She will be playing Hedda Gabler, opposite Adrian Scarborogh as her older (and, as Hedda sees him, rather dull) husband Jorgen Tesman.

The play has been described as “the female Hamlet”, and when you mention that to her, Sheridan’s eyes widen in mock horror, and she briefly puts her blonde head in her hands, and wails

“I know, I know. Thanks ever so for reminding me,” there’s a pause, then she offers: “It makes me shudder to think what I have to pull off, if I’m honest. It’s terrifying. But I like challenges, and I think that I thrive on them, if truth be told. I have to do something bigger, better, rise to the occasion. I’ve always been like that. Why? It’s something inside me that says that I shouldn’t stay inside my comfort zone.

“Self-doubt pushes me on, I’ll admit to that. Can I do it? Am I capable? I might well come a terrible cropper, and make a total fool of myself. Who knows? But I won’t be happy, personally, until I give it a try.”

She’s just re-charged her batteries, by going on a cruise in the Med with her parents, Colin and Marilyn, and brother Damian, although admits she did take her scripts for Hedda Gabler with her.

“That was such fun – and rather emotional as well, because my mum and dad have been together for 47 years now, and they decided to renew their marriage vows – a spur of the moment thing. It was on board in the wedding chapel with the Captain.”

Mum and dad live in Station Road in Epworth, right on the Yorkshire-North Lincolnshire border. Sheridan laughs: “We do have a Doncaster postmark, though, so I can actually call myself a ‘Donny Lass’,” and, as if to emphasise the point, she stands up and goes into a mock-football chant, with her arm raised, and gives a loud “Donnnnn-y – Donnnn-y!”, and then collapses giggling into her chair.

“Come to think of it, Doncaster played a large part in the drama I’ve just finished for ITV – Mrs, Biggs. It’s the story of Charmian Biggs, the wife of the Great Train robber, Ronnie, how they met, their life, and what happened to them after the robbery.

“The actual robbery itself takes only about 10 minutes of screen time, the rest is all about the often bumpy road of their relationship, and how she stood by him through it all. Charmian and I became great friends as we filmed, and a lot of the scenes were done in Melbourne, where she now lives. I was driving to meet her at her home for the first time, and we went past a big sign that said ‘Doncaster’, and I nearly freaked out. I said ‘Whooooo-a – am I dreaming, is this right?’ It turns out that Doncaster in the Melbourne suburbs is where Charmian has her home today. I couldn’t believe the coincidence!”

Shot over several months, the five-part series opens in 1957 when 17-year-old Charmian fails to get a university scholarship, and her stern Victorian father – coincidentally also played by Adrian Scarborough, who plays Smith’s husband in Hedda Gabler – insists that she go to work to earn her bed and board in the family home.

“Charmian and Ronnie met on the train that she took every day, from Reigate in Surrey into London, and the relationship blossomed very quickly,” says Smith. “They fell completely in love, and her father really did NOT approve at all – that’s putting it mildly. He was a headmaster, and insisted on things being done properly. Charmian told me that there were times when he went into his own little internal strop, and never spoke a word to his family for a week or more.

“Charmian was born just before the war, her siblings all arrived after it, and he’d been in RAF Bomber Command, so heaven knows what he’d been through. But he insisted on Charmian calling him ‘Sir’, and he could be terribly cruel – he once told her that as far as she was concerned, ‘You can’t make a silk purse out of a sow’s ear’, and that, to a developing teenager, must have hurt so much.

“I can’t believe that she suffered it all for so long, but when Ronnie came along, it was like opening all the windows of life. My parents couldn’t have been more encouraging, more supportive, more loving. They’ve been behind me every step of the way, ever since I said I wanted to go to dance classes. They always said, ‘Lass, if that’s what you want to do, go for it…’ I am so fortunate in having a family like that.

“Charmian’s father dominated his family with a rod of iron, but her mother finally managed to summon up the courage to visit her when she and Ronnie had their first baby, their eldest son, Nicky. And, for the record, Charmian told me that she had, on a rare visit home, gone to see her father a year or so before he died, so perhaps there was some small reconciliation.”

Charmian and Ronnie’s relationship was a passionate one. “I won’t say that there are ‘raunchy’ scenes,” says Smith, “but there are one or two which tell you how much they really cared for each other. I was sitting in a preview screening room the other day, right next to Charmian, who flew in to see the film, and when those sequences came on, I admit that I squirmed down in my seat a little. But then I felt Charmian pat my hand, to reassure me, which was lovely. Charmian insisted that it had to be honest – warts and all.”

One of the most harrowing scenes, however, is one in which Nicky, by then in his teens, is seriously injured in a car crash, and dies later in hospital.

“I wept when we did it, I wept when I saw it on screen, and if affects me now, just talking about it,”, says Smith quietly. “No parent should lose a child like that – what am I saying? No parent should lose a child at all. I related to Charmian’s grief so much because we’ve had a similar experience in my own family. If I’m honest, when we filmed that scene I had a bit of a wobble on for a few days afterwards... a bit of a breakdown, really. It was a very dark place”.

Smith’s older brother Julian died from cancer aged only 19 and she continues quietly: “Charmian loved – and loves – her sons very deeply. But she told me, ‘You have to have lows in life to appreciate the highs’, and I think that it is so true. In theatre terms, you have to have the flops and failures to appreciate the hits and successes.”

Not that Smith has had many (if any) flops and failures. She started getting attention when she was a member of the prestigious National Youth Music Theatre, and then bounced merrily into the cast of the hit TV sitcom The Royle Family, playing Antony’s girlfriend Emma.

From there, it was a move to Two Pints of Lager and a Packet of Crisps, playing Janet, another girlfriend. Again, the boyfriend, Jonny, was played by Ralf Little. It was in Gavin and Stacey that she met, and had a long relationship with, James Corden. Smith popped up in several other major dramas, including Fat Friends, Mile High and a special Christmas edition of Lark Rise to Candleford, but her name went big, and above the title, when she was cast as Elle Woods in the West End smash, Legally Blonde the Musical. She’d had rave reviews before, but there was nothing to touch the ones for Blonde – she was such a success that she agreed to extend her time with the show, for which she won her first Olivier Award.

Another came last year for her appearance in the revival of Terence Rattigan’s war time drama Flare Path, and she received the Critic’s Circle Award earlier this year, one which, she says “means the world to me.”

She’s now single (“just me and my dogs”) but admits that playing the mother to three boys in Mrs Biggs made her a little broody.

“The boys who all played my sons at various stages in their lives all keep in touch, and send me cards, letting me know how they are getting on, so it’s a bit like being a ‘surrogate mum’, I suppose. So yes, I would like children of my own, but that rather depends on the right man coming along, doesn’t it.”

Charmian met Ronnie on a train – she was reading a Francoise Sagan novel, Bonjour Tristesse. Maybe Smith could find true love in the same way?

She laughs again and says: “On the East Coast trains to Doncaster, perhaps? Well, why not? My reading matter is usually my next script, but, heck, if it worked for Charmian….”

One of the scripts that she’ll pick up after Hedda Gabler has had its run is Shakespeare’s comic classic A Midsummer Night’s Dream, in which she’ll play Titania, Queen of the fairies. Another challenge?

“More like a glutton for punishment. The challenge there,” says Smith with a little smile, “is that David Walliams will be playing Bottom, and I know that I am going to have the devil’s own job keeping a straight face, and not corpsing all the time!”

Hedda Gabblerruns at the Old Vic from September 5 to November 10. Mrs Biggs is on ITV1, September 5, 9pm.