The Game of Thrones star plays a dual role - mild-mannered teacher Simon and his outrageous alter-ego Tracie. Digital Spy and other journalists caught up with Sean to discuss his amazing transformation.
What can you tell us about your character in Accused?
"Simon is this pretty regular, quiet man. I was drawn [to him] in a sense that his life is very ordinary and very structured and very regular - same thing every day. He works at a school as a teacher - he's a very good teacher. Very well read, very erudite. He has a kind of humdrum life.
"And then you've got Tracie, his alter ego, who is very vibrant and big and in your face. Very different, the bang opposite. There's a conflict there - Tracie kind of resents the fact that she's also Simon. You never hear Simon talking about Tracie, but you hear Tracie talking about Simon and his boring life."
So Simon and Tracie are like two different people with two separate lives?
"They've got two groups of people. There's the people in the school, the teachers and the kids and stuff like that. And then Tracie's got a totally different bunch, but they're kept totally separate. They don't ever interact."
Were you nervous at all about embracing the Tracie side of the character?
"A little bit yeah... excited... nervous. I've not played this kind of role before, and I never really thought I would. My agent rang up and said, 'Have you ever thought about playing a transvestite?' and I said, 'Not really, no!'
"It's a Jimmy McGovern script, which is exciting. I've always liked his stuff, I'm a big admirer of his work. She sent me the script and I read it and I was really impressed by it. It's a very moving, unusual story.
"So I kind of knew I wanted to do it, but it was quite a long process reaching the character that I wanted to portray. Psychologically, it was quite a leap. A big leap. That takes some time to get to - it wasn't just... bang, switch it on. You can't just suddenly turn yourself into a character like that.
"It took a while, with how you carry it physically and also psychologically to get my head around it. That was quite a journey. I thought I'd got it, then I hadn't. By the first day I just had to jump in feet first. Once I'd done that I'd knew I'd cracked it and felt good."
What kind of research did you do before filming?
"I watched a load of documentaries. At the time there was [Channel 4 series] My Transsexual Summer on television. I also went out in Manchester, because there's quite a big scene up there - transvestites in Canal Street. I talked to people, went out on Saturday nights and just had a drink and observed and chatted. That was quite revealing and quite intriguing.
"I think we have a perception of transvestites all being the same, as one block. It's not one mass or tribe. Everybody's got a different story. I thought that's something I never knew before - you just categorise people."
It's a topic that's not explored often on television...
"No, not really. You never go that deep or explore that intimately. It's either comic or it's really serious. This really hits the nail on the head. It's full on, it challenges the audience. It gives you an insight into it."
What is Tracie's journey throughout this Accused episode?
"It's about joy, really. Joy and happiness, this is what Tracie's trying to find. She's desperate. She never imagined it would ever come her way and it does in the form of Tony, Stephen Graham's character.
"It's about two people, totally different, that have this connection, who love each other. As an audience, you kind of get drawn into that. You empathise with them and their dilemmas and their situation."
How do Tracie and Tony meet?
"The first time they meet is in a bar in Manchester and Tony is out with his brother on a stag night and a bit p*ssed. I walk in all dressed up to the nines and his brother starts having a go at me. He's a nasty piece of work - he's swearing at Tracie and it's just a barrage.
"There's a row and... I'm good at coming back because I've had it so many times over the years. She's very good at deflecting criticisms. She walks out and they follow her and she's scared. That's when Tony intervenes to stop his brother from getting into a fight."
Can you talk about the physical transformation you had to undergo to become Tracie?
"It was just kind of dressing up. The waxing stung a bit! I had almost a full body wax, and that was quite weird. I had my toenails painted red.
"That was a good thing to get you into it, the wax. You feel different. The transformation of putting the make-up on and the wig and everything is kind of a ritual. That channelled you into the part. It helped in the mornings, it was kind of ritualistic. It channelled me into her.
"The shoes were the worst, the high heels were more painful. I practised quite a lot, just wearing them in the kitchen, in the house. I just wore them and it was hard. It's painful. I don't know how women do it every Friday [or] Saturday night. They're dancing, it's hard.
"I've never felt before so much that I was jumping into something. It was scary but exciting at the same time. Yeah. I just went for it really, there's no other way to do a part like that. I got there!"
Jimmy McGovern is one of the most respected writers working right now. What do you think it is about his work that is so special?
It's sort of truth. It's unashamed work. It's realistic... he knows working class people. He knows what gets them going, how they react, their emotions. He says that himself, he says there's no point in him writing for Downton Abbey. He knows how to portray these [working class] people.
"I think that's brilliant and you don't see it very often. He just goes for it full on. Everybody wants to work with a writer like that. He has so many stories, there's no loose ends. He's very raw. You just go into it, you don't have go through all the gears - you're in top gear already."
Each Accused character ends up in the dock - how does Simon / Tracie get there?
"He's implicated... you're not sure if he's done it. It's told in flashbacks."
Do you think UK TV can match the quality of American television?
"Jimmy's been doing this for years and it's fantastic stuff. We don't have as much as they do. They've got more money - they've got big networks with a load of cash.
"We haven't, but what we do get is stuff like this. I think Red Riding that I did and other stuff... that's all great television."
Looking back... do you feel like there's one role in particular that has defined your career?
"I played Richard Sharpe [on ITV's Sharpe] for years, over a period of about 14 years on and off. That stays with me - people relate to that and see me as Sharpe in some sense.
"I've had the chance to diversify and play other roles like Boromir and Red Riding - I always find it a joy to see what's thrown at me. I don't think I've ever had a real desire to spick out any particular role - I just see what comes up."
What do people approach you about in the street?
"Sharpe, I suppose. People like Sharpe - cab drivers and stuff. Lord of the Rings... in America a lot of people come up about Sharpe, which is surprising. Sharpe was a bit tongue-in-cheek, but you don't see that in America."
Did anyone recognise you when you were out as Tracie?
"Not really no, they just walk past. There's confusion about whether it's me. The show's not all doom and gloom, there are moments that come out of the situation. When I went it to the beautician, I said, 'I want to look like Cheryl Cole' - it was a bit of a laugh!"
Accused begins on Tuesday, August 14 at 9pm.