Stacey Dooley has just turned 30, but in a decade has already made a staggering 60 documentaries for BBC3. Her freshness, warmth and bravery have won her many fans and some detractors – her style is, by any standard, unconventional. To some, she might seem like the girl next door who has left home to travel the world and is amazed by what she sees. Her eyes are wide as she asks what appear to be innocent questions with results that are often unexpectedly illuminating.

In On the Frontline: Girls, Guns and Isis – a harrowing film – she investigated Yazidi women fighting Isis in Iraq to avenge their sisters. The courage involved in going close to the frontline at a point when Mosul was yet to be liberated was, in itself, remarkable. She has also interviewed child soldiers in the Congo, prisoners in New York and underage sex slaves in Cambodia. She has investigated the crystal meth industry in Mexico, the desperate plight of women in Honduras and schoolgirls caught up in Japan’s sex industry (she was arrested by police for asking questions on a Tokyo street). Closer to home, she recently made a revelatory programme about the UK’s digital drug industry, exposing a “seismic shift” in the way drugs are being sold by children via Snapchat and Instagram.

Her latest documentary, The Billion Pound Party, looks at the DUP in Northern Ireland, and is currently one of the most-watched programmes on iPlayer. She discovered that many DUP voters do not support the party’s anti-abortion or anti-gay marriage policies. Their voting has been tactical: to keep Sinn Fein out and Britishness in. She is pleased with the “tons of feedback” the programme has received. There was huge opposition from the DUP to its making – graffiti on the walls of Belfast, telling the BBC to get lost. Sinn Fein and the DUP are now “pissed off”, she says, which means, “we probably got it right. If you piss off both sides, you’re somewhere in the middle, which is where you want to be.” And she gives her wonderful, defusing laugh, useful for a journalist.

Dooley is always talked about as an unlikely person to be making hard-hitting programmes. She tells me breezily that her “USP is sympathy”. In another interview, she said it was ordinariness. She grew up in Luton and left school at 16, having no expectations of a media career. She is tiny, with a chirpy manner, and shows up at the Observer’s London offices wearing a checked waistcoat and no shirt – cool in every sense with a mane of blond hair and obvious presence. I find myself instantly resisting the “ordinary” argument. After all, what Dooley has achieved so far is extraordinary. Her programmes cover an astonishing range of subjects and there is, in all the documentaries I have seen, a sense of getting under the skin of the people she interviews. She modestly points out that her programmes are character-led. She leaves her own character out of it, but she is their crucial ingredient.

Does she feel the odd woman out in the male-dominated, Oxbridge-educated world of broadcasting? “It’s something I’m proud of,” she says, “although some people give me a hard time and tell me to ‘speak properly’. Some don’t like my accent or the fact I’m from Luton, but we often beat middle-class, middle-aged men in the ratings.”

Stacey Dooley joins the Yazidi women for their morning training session in Iraq. Photograph: Almudena Garcia-Parrado/BBC/Insight TWI

It does not bother her that her documentaries are not premiered on the more grownup BBC channels. (Since February last year, BBC3 has been online only as part of a £100m cost-cutting exercise, though earlier this year won channel of the year at the Royal Television Society awards). “I’m not toeing the line when I say BBC3 is an incredible channel – it has evolved and found itself,” she says. I ask about the recent pay-differential row and she replies: “It does not affect me personally, although I’ll always be pro-equality and equal pay.”

Equality matters, but what motivates Dooley most is curiosity. She was brought up by a “vocal, opinionated” mother and a stepfather. Aged 20, she was working at Luton airport in the perfume and makeup section of duty free when she was chosen to be a participant on the BBC3 series Blood, Sweat and T-Shirts. She went to India, “completely naive and unaware”, to work alongside Indian people making clothes for the UK high street and was horrified by the child labour she witnessed. She took in the acute poverty, the unthinkable 15-hour days in a sweatshop, the hunger – and was knocked sideways. If you revisit the footage, you can see where her career started. She kneels opposite a little boy and talks to camera: “He’s a baby and he got beaten up by a grown man – it makes you sick.” She is crying unstoppably. It is impossible to watch without responding with emotion – her empathy is a such a force.

Returning to Britain, Dooley campaigned against child labour, appeared on Newsnight and was approached by Danny Cohen (then the controller of BBC3 and a “true gent”) who commissioned her first documentary. He advised her to use her inquisitive empathy and not mimic other journalists. Most people find it hard to be natural on television. Not Dooley. Part of the pleasure of watching her is that she often looks as incongruous as a tourist who has strayed into her own documentary by mistake. More mature nowadays, her eyes still fill readily with tears – a challenge to the tradition of “neutral” reporting. Yet she never seems unprofessional. Her reactions are truthful and involved.

If you tried to avoid people telling you to eff off, you’d never get a decent interview

Sometimes, she touches the arm of the person she is addressing to reassure them (“I’m tactile,” she tells me apologetically). When she nods her head and says: “Got yer”, she might seem an acquiescent airhead, easy to dupe. You might worry that she is looking impressed while interviewing a group of masked drug-dealers. But actually she is about to spell out alarming facts; to let us know how easy it is to get hold of class A drugs in this country, to reveal that some dealers are as young as 13. Wherever in the world she fetches up, she is always a defender of children’s rights.

It is perhaps with this in mind that she asks a Glaswegian drug dealer (extraordinary he agreed to be filmed) how he would feel were his six-year-old given heroin. He says he would beat the dealer up. Head on one side, Dooley tends to ask her most difficult questions slowly, proceeding at a gentle, confiding, non-incendiary pace. “Can you see the hypocrisy there?” she wonders. Her face itself is equivalent to a question, her features arrange themselves into agenda-free wonder. She never makes the mistake of looking as though she has the answers.

But Dooley is anything but clueless and some of the most powerful moments are when she tells you what she thinks. She is unafraid of grasping moral nettles and does not give up on pursuing elusive interviewees. She made a documentary about Muslim extremism and the growth of the right wing in her hometown of Luton and had to work hard to get an interview with Stephen Yaxley-Lennon (aka Tommy Robinson), leader of the English Defence League, whom, she revealed at the start of the programme, she knew growing up. Eventually she put him on the spot. She also talked to Muslim girls she was at school with and there is a particularly touching conversation filmed in a mosque where you briefly entertain the illusion that if this handful of women were in charge, they would eliminate conflict. More challenging is her encounter with two Muslim extremist men accusing her of naivete, who tell her, in a confrontational tone, that the way she is dressed is indecent. It is a programme in which she is a potential bridge between enemies. It is powerful because it is about the importance of listening and the impasse reached when no one will hear what the other has to say.

Dooley has learned to avoid “polite and mundane” questions. “You have to be brave and ask the questions on the tip of your tongue. Sometimes, you’re cringeing inside. But you can’t have people screaming at the screen the question you didn’t have the balls to ask.” Later, she adds: “People can tell you to eff off and do all the time. If you tried to avoid people telling you to eff off you’d never get a decent interview.”

Stacey Dooley speaks to gang members for a film investigating children selling drugs online. Photograph: BBC/Time Inc UK Ltd

The question she is asked most often is about her safety – how often has she been afraid? “A handful of times. The most difficult situation was in Northern Iraq, we were so close to the frontline.” I was terrified for you, I say. “You and my mother,” she laughs. “It’s surreal… Half of you is desperate to be there because as many people as possible need to understand these girls; the other half wants to run in the opposite direction. When you hear shots, you think, ‘My worst nightmare is going to happen’.”

Dooley’s ability to put people at ease is one of the keys to her success. There is a harrowing interview in her film about the Japanese sex industry in which she wins the trust of one of the schoolgirls – a Japanese Lolita, interviewed in the dark. The girl begins by boasting that she earns £425 per customer, then suddenly drops her guard, revealing: “I do it as a form of self-harm. I have suicidal thoughts.” Dooley looks on aghast and wordless. One of the unusual things about her as a reporter is how often she is rendered speechless. In the film about the Yazidi girls, she exclaims: “It’s beyond belief.” Words – visibly – fail her.

But she makes personal connections. Thinking back to that film, she tells me she wishes a GoPro could have captured the night the Yazidi girls pulled her into a room to dance with them to music from one of their mobile phones. They were “laughing, jumping up and down, taking selfies. It was a precious time.” That film was made almost a year ago. Dooley tries to keep in touch and hears that the girls are doing reasonably well – they are still alive at least. Some of their family members have been rescued from Mosul, although many have suffered mental and physical injury. She knows the story doesn’t end with the closing credits. She says, wistfully: “We had more in common than we didn’t. They’re just normal girls.”

And what of her own life? How normal is that? “Now I’m 30, I’m starting to think I’d love a family. I think it will happen one day. But now is not the moment. I’m enjoying my job so much. I feel established, comfortable and, after 10 years, I have the authority to have an opinion.” It must be tiring? “It can be mentally draining, though you feel so effing middle-class moaning about travelling the world when it is a luxury.” She is super-conscious of her fortune compared to many of the people she interviews: “I’ve always understood I’m privileged.”

And what’s next? “Florida,” she says. She is visiting Miracle Village, – a sex offenders’ community. Does she ever feel disingenuous, smilingly interviewing people of whom she disapproves? The sex offenders appal her, she says, but it is important to approach everyone “on a human level”. Whoever she is interviewing, “you have to stay sincere to yourself, but allow them to speak. And you must never assume you know what they are going to say.” And then she adds the simple thought, typical of her ability to home in on what counts: “Hate won’t solve anything.”

Stacey Dooley Investigates: The Billion Pound Party is available on BBC iPlayer