And yet, playing these wretched unfortunates has never got the actress down. In fact, quite the opposite. "It never depresses me," she says. "It's actually exhilarating. I've always found these parts therapeutic. You go in, have a good cry, and then go home again feeling fine. It's better than therapy. Perhaps that's why I've never needed therapy, because I've been cast in all these parts where I've had to let it all out."
And the more she lets out, the more audiences appear to lap it up. They know an authentic portrayal when they see one. "People seem to relate to this class of middle-aged mother. One of the greatest joys I had was the response from people with Motor Neurone Disease when I did Goodbye Cruel World. It was the same when I played a rape victim in Brookside. It's great if people are able to identify with this woman who is not particularly glamorous, who goes to Tesco's and who leads a life that is no different from theirs. You hope you can somehow touch their lives by showing them something like Motor Neurone Disease in as real a way as possible. I like to be down-to-earth about things. I'd hate to be a glamoriser." She's very far from that. Her enviable reputation for gritty realism stems from her seven years suffering in Sheila Grant's immortal, white-spotted pink jumper. With the endless misery she had to endure in C4's soap, she became a virtual icon. "I heard about someone recently saying that the She in Sheila stood for `all women'," Johnston smiles. "Sheila struck a chord with an awful lot of people who saw her as a strong, fighting mother having to cope with suffering. She was a gay icon, too. Like Pat Phoenix in Coronation Street, she had that `I Will Survive' side."
Unlike many soap actors, Johnston had the good sense to quit while she was ahead. "I started getting bored on Brookside," she admits. "I felt they didn't know what to do with the character. Once her affair had settled down, it was like, `Where do we go from here?' She became a religious nutter, Saint Sheila, and I couldn't see where it would go. When John [McArdle] left, I thought, `Sheila can't lose another guy and go through another trail of tears'."
So in 1990, Johnston bravely left Brookside Close, aware that snobbery was prevalent about actors seen to be still dripping soapsuds. "There was this thing of typecasting. I was amazed at some of the people who came out of EastEnders and didn't do so well. Nick Berry has been astonishingly successful, but why hasn't Anita Dobson, who is a thunderingly good actress? Did casting agents feel they couldn't risk putting her in things because she was too strongly identified with Angie? But it is possible to do things after soaps."
Johnston proved as much, almost immediately, when she starred with McArdle in the well-regarded stage-play, To, by Jim Cartwright. "At Edinburgh, we were petrified on the press night. We imagined the critics sitting there thinking, `OK, you're soap actors. Let's see what you can do'."
She needn't have worried. The producer of Goodbye Cruel World was in the audience for To, and at once snapped her up for Tony Marchant's award- winning BBC2 drama. "Tony and Jimmy McGovern are the most extraordinary young writers.They'll thank me for calling them `young' - creep, creep... Goodbye Cruel World was tenderly written, without being over-sentimental. It was harsh, not pretty, and didn't spare your emotions. You felt as though you were doing something which meant something. It wasn't frivolous."
For all the heaviness of her roles, the Warrington-based actress is light- hearted in person. As we sit by the set of her latest play, Sugar, Sugar, at the Bush Theatre, she points to a full aquarium at the back of the stage. "The fish are wonderful," she deadpans, "they never forget their lines." Later she asserts that "the size of a part has never really interested me - I'm talking about plays, not men."
We will soon be engulfed by what amounts to a Sue Johnston festival. She is continuing in Sugar, Sugar, Simon Bent's black comedy set in the Seatos guesthouse in Scarborough. "It's an intriguing slice of the life I know only too well. It's about the danger of repressed emotions, and the way people talk at each other without listening. It's a cross between Joe Orton and Harold Pinter."
Much as she is enjoying it, the 54-year-old Johnston is finding the demands of theatre tough. "I have to discipline myself, which is why I get so mad with Gazza. He gets paid a fortune and still can't focus. And here we are at the Bush, focusing ourselves like crazy for pounds 200 a week."
In addition, Johnston is co-starring with Richard Wilson in ITV's new sitcom, Duck Patrol. "It's very Sunday night," she explains. "It won't be up everyone's street, but it will be popular in that slot. When we were young, there used to be interludes on television. One was a potter's wheel, and the other was a boat going down the Thames. This is like that - only more interesting than an interlude."
She is also playing a hard-nut prosecuting counsellor in Verdict, ITV's new prime-time reworking of Crown Court. "People have a fascination for courtroom drama. Whenever the soaps go to court, people love it. And look at the reaction to the trials of Louise Woodward and O J Simpson. It's like peeping behind the curtains of people's lives. We love feeling smug and safe while other people go through the most awful things."
Later on in the autumn she will also be seen in The Jump, a fast-moving ITV thriller, and The Royle Family, a new sitcom for BBC2, in which she and Ricky Tomlinson (her first screen husband from Brookside) play Caroline Aherne's parents.
With all this exposure, the actress is worried about Johnston Jaded Syndrome among viewers. "You'll get bored of me," she sighs, before adding, with a laugh: "But I'm very different in every single thing - only my nose is the same."
`Sugar Sugar' is at the Bush Theatre, Shepherd's Bush Green, W12 (0181- 743 3388) to 15 Aug
`Duck Patrol' is on Sundays at 8pm on ITV
`Verdict' begins on Friday at 8pm on ITV