To be perfectly honest, Chris Langham's new film was the last thing on my mind when this interview was first arranged. The drama of his private life has eclipsed his identity as an actor so entirely that I barely gave the film a thought – except to wonder whether it would ever be possible to watch him again without being distracted by memories of the scandal that banished him from our screens. What cinematic tale could ever hope to compete with the true story of Langham's crime and punishment? If an actor can't persuade his audience to suspend disbelief, then he is not really an actor – and Langham has not acted since his arrest in 2005 for downloading child pornography, since when he has been vilified in the tabloids, spat at in the street, imprisoned, unemployed and broke.

Less than five minutes into Black Pond, I'd forgotten the lot – and remembered how extraordinarily gifted Langham is. His performance is comical and heartbreaking, utterly absorbing and unnervingly naturalistic, like the best of Alan Rickman and Steve Coogan fused into one – and the film is a total delight.

It's too stylish to feel like a low-budget movie, let alone one written and directed by a pair of 25-year-old novices, one of whom – Will Sharpe – used to be in Casualty. But the budget was so tiny they shot it all in a fortnight in Sharpe's parents' house, while Langham lodged with the family; when they filmed a scene in the bedroom, Sharpe's father was outside the door in his pyjamas, waiting to go to bed. Langham is worried that if I say how much Black Pond cost to make, people might be put off – so maybe it's best not to know until after you've seen it – but for that money you'd struggle to buy a mid-range family car.

The last time Langham appeared on our screens in a professional capacity, he was winning best actor before a glittering celebrity audience at the British comedy awards in 2005. His stock had never been higher. The star of The Thick of It, which won best new comedy that night and went on to become the hit movie In The Loop, Langham was also enjoying delirious acclaim for Help!, a comedy about therapy co-starring Paul Whitehouse. Two days later news broke of his arrest, and he hasn't worked until now. "The point of being in this business is the work. It's not about being a celebrity. The reason why you work is to be allowed to work again." But ever since Sharpe cast Langham, people in the industry have been telling the director: "Everybody wants to see me working again – and nobody wants to hire me."

So the stakes could scarcely be higher for Langham right now. His future will depend in large part on whether or not viewers believe he is sexually attracted to children. It would take far too long to revisit all the details of his trial, but in brief Langham was caught with 15 graphic videos of extreme child abuse on his computer. He has always sworn he downloaded them in order to research a character for Help! who would be a paedophile – and that he could only bring himself to watch a few seconds of four of them. Expert witnesses testified that he was not a paedophile, and appeared to convince the judge, who told Langham: "Paedophilia is not an issue in this case. You are not a sexual predator." But a police officer told reporters outside, "I am satisfied that he is a paedophile," and either way he was guilty of the offence of downloading images, and sentenced to 10 months, reduced to six on appeal, of which he served a little over three.

I spend an afternoon at his family home in rural Kent, and for what it's worth – because of course, we'll never know – I believe his explanation. I've never heard of a paedophile downloading just 15 images – their computers usually contain thousands and thousands – and nothing he says makes me suspicious. But he's more forthcoming with his emotions than anyone I've ever interviewed, to the point where he seems practically naked, and I feel as if we're in therapy. He cries so many times I lose count. Every now and then the intensity lifts – "You talk to people in prison, and you listen to them talking, and you think, fucking hell, he should be locked up! Oh yeah, he is," – but his vulnerability is always there in the watchful tension of his gaze, and the way he hugs himself with folded arms. Yet I'm still not sure how he really feels, because each answer depends on which version of him opens his mouth. There are two Langhams, and he keeps switching between them as if I'm interviewing two entirely different people.

One version must be desperately anxious, because he tells me: "I worry to the best of my ability, almost constantly I worry. It's one of my chief skills. I do it at almost every available moment. It is like a narcotic sort of thing." For example? "Oh, waking up in the morning, oh my God, another day, terrifying. That's sort of me."

His problem, he says, is "I'm desperately hung up and dependent on people's opinion of me. I've always been like this, I don't know whether I'm OK until I find out whether you think I'm OK or not, because you outrank me. You have the authority to decide whether I'm a good or bad person, and I don't." Does he want my approval right now? "Probably, yeah, I think so, yeah." His other problem is: "I am what I do. I'm only as good a human being as my last job. If my last job was praised, then I'm a good person. If they thought it was less than good, then I'm a less than good person." Not surprisingly, his arrest reduced him to "some kind of Dickensian character, walking around in circles going, I'm ruined, I'm ruined". The 18 months before his trial were the worst. He came close at times to suicide, "and was quite unhinged".

But then the other Langham says, "This has been a fantastic time, really wonderful, and I'm grateful for it." Partly that's because he's spent six years at home with his family, instead of being the kind of father who thinks he's great, but tells his kids: "I'm really sorry I can't be there at your prizegiving, I've got an awards ceremony of my own to go to, which is obviously more important."

But mostly, he says, "This sounds insanely sort of Pollyanna-ish, but I got sent a situation that couldn't be more perfect. Everybody in the country hates me – and I can't get a job. So if I wanted to resolve those two issues for myself then I couldn't have asked for a more beautiful arrangement than what happened to me." Has it resolved them? "Yeah. Now if you don't like me, it's fine." Genuinely? "Yeah, it's really fine. It's fine. Cos you know what? I don't like me most of the time. And so I guess we agree."

I'm not sure we agree at all about what he says next. "What I'm saying is that when I look back, nothing in my life has ever gone wrong. Ever. Nothing. Ever. It just happened differently to how I'd expected it to be."

That's going to make you sound mad, I start to say – "But it was appropriate!" he interrupts. "I did something wrong and I got sent to prison. It was appropriate. Some people lied about me as a result of all that happening. That's not anything going wrong in my life. That's something going wrong in their life. If they want to tell lies about me then that's their stuff."

I tell him I'm struggling to work out how much of this is real, and how much is a story he's told himself to make life bearable. "Yeah, that's pretty good, I've told myself a story that's made my life bearable, and I've found out that it's a true story. The trick is, whatever you believe turns out to be true."

Perceptive readers may by now have guessed that Langham belongs to Alcoholics Anonymous. Born in 1949 to theatrical parents, his early career was precociously successful; the sole British writer of the The Muppet Show in his 20s, he then became a member of the original Not The Nine O'Clock News team. Unfortunately he also became a rampaging alcoholic and cocaine addict, and was sacked after series one. His first marriage – from which he has three adult sons – collapsed as he unravelled into an unemployable mess of narcissistic fury and crippling fear, and eventually he wound up in AA.

Still sober 25 years later, he attends four or five meetings a week. So it's hardly surprising that he's fluent in the 12-step philosophy of serenity, and keeps insisting, "I really do live a day at a time, and it's a great way to live." If alcoholism is the defining motor of his identity, as he says, then AA has unquestionably saved his life. But when he says: "I can decide my life went wrong if I want to. But what's the point of thinking like that?" I can't help thinking there's a fine line between serenity and denial.

I think Langham knows that, too, because when I ask when he's been happiest he has to think before appearing to remember the right answer: "Ummm … Oh, today!" The other version of him finds it easier to admit, "Not being able to work is terribly painful, because it's what I do. But the tabloids are a bit like the secret police. Everyone despises them but everyone's terrified of them at the same time, and so everybody was frightened of professional association with me, in case some tabloid said you're obviously soft on paedophiles.

"Because that's the big lie – that I am one. It is a lie. That's one of the things that I've struggled with forgiving. The judge stated really clearly that I'm not a paedophile – and then the policeman said afterwards that I was – and that's what the press have run with ever since."

Didn't he realise what he was doing on his computer was unbelievably dangerous, and indefensible? "I knew it was illegal. It was just hubristic and arrogant of me to think I'm above the law because I'm an artist. I thought after [I'd played the character], I'll reveal I'd researched it, and people would say gosh, you've taken this really difficult subject and done something amazing. I'll do this thing, and everyone will think I'm wonderful. I thought if I pulled it off, everyone would admire me. And I'm a slut for approval."

During the trial he revealed that he had been raped by a family friend when he was eight. "But until the trial I'd always felt ashamed about having been abused. What is seared in my memory is lying in bed with this guy with his arm round me afterwards, and he told me that I'd done well. And I was so relieved that he approved of me. And at the same time, at just eight years old, I thought, fucking hell, are you this much of a slut for a kind word? Would you do this much for a kind word? And I'd carried that all my life."

In Langham's mind, it is all connected – the rape, his craving for approval, the alcoholism, and the arrogance, which was just camouflage for shame. That's not how others saw it. "After I came out of prison I went to the Co-op, and it was like a really big event for me, and this little old lady tottered towards me as I was walking in, and she looked like she was going to say something really nice. Just a lovely old lady, you know? And she just leaned into me, and she said: 'People like you should be killed at birth.' But then, you know, she's only operating on the information she's been given by the newspapers she's read." Did he have to remind himself of that to feel better? "Actually," he says, slowly smiling, "I remember my first thought was, how would that work? How would you actually put that into practice? I can't quite get the system here." Then he looks concerned.

"I feel like a lot of the time I'm talking I'm justifying myself, but I'm very sorry that I offended people by what I did. And particularly sorry to have offended other victims of child abuse. That was the last thing I'd have ever wanted to do. And I'm really sorry that because of my thoughtless, wilful, arrogant behaviour, I created enormous difficulties for my wife and my children, who I would do anything to not hurt."

He never feared his family might not believe him. "That never even occurred to me. As a family we don't have secrets; we only have two rules in our house, tell the truth and be kind." He has a daughter and son with his second wife, a musical director, who were 11 and nine at the time, and the couple took advice from child psychologists on how to help them cope – but he was forbidden to be alone with them until he went to trial.

In court he was also charged with sexual assault on a minor. When news of his arrest broke, a young woman had come forward claiming they'd had a sexual relationship when she was 14. Langham admitted to the affair, but said she was 18 at the time, and her evidence was dismissed as unreliable, with court reports describing his accuser as "troubled". He says his wife had already known about the affair – but if the woman was so troubled, why did he get involved with her?

"I don't know," he replies in a low voice. "I don't know. I don't know. Cos she was attractive. I've made lots of mistakes in my life and that's one of them and I'm very sorry that I did that."

Other than his family, Armando Iannucci, Mel Smith and John Cleese all publicly stood by him. "There weren't many more. But – but – well you know, you don't need more than a few friends. And I've got a few friends. So I'm blessed." It's only thanks to friends' generosity that he didn't lose the family home.

Acts of kindness move Langham to tears, and before long another memory has him sobbing. A tradesman arrives and hovers awkwardly in the hallway looking alarmed, but Langham isn't the least bit embarrassed. "I leak all the time," he laughs ruefully, wiping his face and going off to talk about solar panels.

Afterwards we go outside to do the pictures, and I try once more to unravel the paradox of his dual selves – one version frightened and hurt but still hopeful, the other glassily zen and indifferent to his fate. Can both be authentic? "You keep saying, when did you stop thinking like that and start thinking like this?" he struggles to explain. "But it's simultaneous. This is an example of it right now," and he points to the flowerpot the photographer has asked him to hold. "My head is screaming, this is no good, you're going to look like a complete jerk! Why are you holding a stupid flowerpot? But I now have another voice which goes, you have no control over this, but it's going to be fine."

He is currently writing a comedy with Sharpe for a production company who say they love it. They just don't know if a channel will be ready to broadcast Langham's work. "If people think I should do longer of not being allowed back in the room, as it were, then that's not my choice, it's their choice, and I have to respect it. Expectations are just planned resentments in my experience, so I don't do expectations any more. Some people will think it's probably not an idea to be in my orbit until the dust settles and it's all right."

Does he think it's all right yet? After a long silence, very quietly: "No, I don't think so." Will Black Pond be a step in that direction? "I guess so. I hope so," he admits, and tears well up again. "Because I'd like to be able to support my family."

• Black Pond premieres at the London Raindance film festival on 2/3 October, and is released at selected cinemas on 11 November. For tickets and information see blackpondfilm.com