Linda Gray says there’s a simple reason why she’s reprising her iconic role as Sue Ellen in Dallas: “I was invited,” she smiles. “Maybe it has happened before where an actor gets to play the same role 20 years later, but there can’t be many.”

Gray returns to Dallas, along with Larry Hagman as JR and Patrick Duffy as Bobby, two decades after the quintessential 1980s soap opera came to an end (apart from a couple of forgettable made-for-TV movies).

She is obviously relishing the chance to play the role again, as well she might. In the words of Sue Ellen’s errant husband JR, her character was “a drunk, a tramp and an unfit mother”, who suffered magnificently thanks to the machinations of her men. As Sue Ellen once described it, “being married to JR is like a Hitchcock movie. You start out laughing, but soon find yourself screaming in terror.”

With her lips permanently glossy and her hair only out of place when she’d had a bad night on the bottle, Sue Ellen brought Gray a global fame that has endured. In 1980, the cliffhanger “Who Shot JR?” made news around the world. The revelation of who actually fired the gun was watched by 83 million people in the US alone.

“I don’t feel as if Dallas ever left my life. Over the past 20 years, so many people have had stories about Dallas and I hear them daily.”

Gray says this without a hint of the weariness that some might display in the face of relentless fan adoration. Even so, deciding to revisit Sue Ellen, in what’s being billed as a reboot rather than a remake, can’t have been simple, can it?

“Cynthia Cidre [the new executive producer and writer] calls Larry, Patrick and myself ‘the big three’ and our friendship was significant in coming back. We trust each other, we respect each other, we know how hard each of us has worked to maintain a family life – we're all grandparents now. We’ve seen each other through deaths, divorces, the birth of children, all kinds of things. We’ve been a family on-camera and off. The reality of having friendships for 33 years is extraordinary to me.”

Patrick Duffy agrees that the closeness that he, Gray and Hagman share was key to any reunion. “I’m closer to Larry and Linda than I am to my real sister, who I love completely. So the reunion was the characters’ and not ours,” he explains.

“We live in different bits of the country now – Larry lives in California, I live in Oregon and Linda lives way out in the country – but it’s an unspoken rule that whenever we’re near, we have lunch or dinner. And we call and text. Everything we do connected to Dallas, we make a collective decision. If Larry had turned down the show, I don’t think it would’ve been done. But had they tried to, Linda and I wouldn’t have done it.”

Duffy’s deference to Hagman is unusual in a business where ego normally dictates that an actor of stature defers to no one. But then Hagman’s JR – who initially wasn’t the star of the show at all, but a supporting character whose womanising villainy quickly endeared him – is one of those characters known even to those who don’t watch television. His quips are legendary: “Monogamy is not exactly second nature to me, so I want full credit for my efforts”; “I’ve never seen a woman open her mouth more and say less”; “I’m JR Ewing – I don’t get ulcers, I give em!”

So how does the 80-year-old Hagman feel about reviving his role? “JR is one of the most famous roles in modern history and to get the chance to do it a second time is wonderful,” he says. “I thought it was an opportunity to have fun, make money and re-establish myself as an icon.” Besides, Hagman says, “Just to have a job at 80 is wonderful. How can actors retire? I’d like to die on stage, so to speak.”

But it hasn’t all been plain sailing for Hagman. After starting work on the show, he discovered his cancer had returned and had to undergo treatment during filming. Even for a man half his age, that would have been a challenge, but Hagman insists, “It wasn’t tiring, just time-consuming going to the doctors all the time. It worked out pretty well and I’m feeling great now.”

Having had a preliminary all-clear, Hagman has his eyes on the future, but isn’t under any illusion as to whether this new Dallas can replicate the success of its predecessor.

“I don’t know if we can do it again, but we can give it a good try. The reason it took off then was that there was a big recession in the 1980s. People couldn’t afford to go to the movies and get a babysitter, so they stayed at home and watched Dallas.”

The spectacle of rich people behaving badly might strike a chord once more, then, in austere times. Hagman assures me that JR hasn’t changed a bit. “He hasn’t mellowed at all. He’s got a little slower, but he is still a son-of-a-bitch," he cackles.

For Gray, by contrast, it was vital to know that Sue Ellen had changed. Brace yourselves, Dallas fans - there’ll be no sign of a sozzled Sue Ellen any time soon. “I feel Sue Ellen has been there, done that and finished that chapter. I got bored of it. And when an actor gets bored, it translates onto the screen. So 20 years later, I didn’t want her to be drinking, and I was going to be very verbal about it if she was.” Luckily, the producers agreed.

“There were a few women on the original show, but no one at executive producer level. I was excited our business had changed. It’s still a male industry, but there are many more women executives bringing strong female characters to the screen - though we still have a long way to go."

Gray identifies Britain as leading the way in employing older actresses. “I applaud the UK because you embrace older women much more than we do, and there’s such a life-force energy that comes when you keep on working.”

For Gray – as for Duffy and Hagman – returning to Dallas was akin to returning home. And Gray had an added incentive, as, for the first time ever, Sue Ellen gets to hit JR.

“It was supposed to be a secret,” Gray says giddily, “but Larry let it slip. It was wonderful, but they had to teach me how to do it properly because I had no idea. So that’s really good fun because, you know, JR certainly deserves it.”