Most news anchors are inevitably not quite their real selves in front of the camera. The voice is a bit more raised, the pose more self-conscious, the persona more weighted with gravitas.

Last Wednesday afternoon, in a TV studio in mid-Manhattan, Christiane Amanpour was rehearsing her evening talkshow, Amanpour, which begins tonight, and will be shown at 8pm and 10pm on weeknights on CNN International. It will mark her return to the network that was her home for 27 years before she left in 2010 to host ABC's This Week.

Such is Amanpour's clout that she has managed, for the second time in her career, to negotiate a deal that will allow her to work on two networks simultaneously (from 1996-2005 she was signed to both CNN and CBS's 60 Minutes.) She will still host occasional news specials for ABC, but CNN could not be more ecstatic about her return, rewarding her with a primetime nightly spot on CNN International.

Amanpour sits at a round table, smoothly moderating between her guests, the historian Simon Schama and the Wall Street Journal columnist Bret Stephens. After about 20 minutes, the cameras turn off and Schama and Stephens visibly relax. But Amanpour is utterly unchanged, not even altering her posture, and she continues to converse so seriously with her guests that they look a little confused as to whether or not they're still on air. The combination of erudition and curiosity, commanding glamour and chatty accessibility that has helped to make Amanpour such a popular figure for liberal news audiences and networks is no mere TV persona.

"I feel so energised to be able to reconnect with a global audience," she enthuses when we meet in her office in Time Warner's mammoth HQ. The office itself is poky but with extraordinary views over Central Park and it's tempting to see it as a metaphor about CNN itself – a network retaining an ambitious global outlook but looking a little anachronistic. In America, CNN lags behind other cable news channels such as Fox and MSNBC, so her return could not be better timed.

It is also well timed for Amanpour. Her stint at This Week (which is a little like The Andrew Marr Show in terms of its Sunday-morning slot and prestige) was not the kind of roaring success she has usually enjoyed throughout her career. ABC slumped from second to third place in the Sunday-morning ratings and her international outlook did not always feel a comfortable fit with a show that had always focused on Washington DC politics.

"To host This Week really is an incredible opportunity, and it would have been dumb to turn it down, and I went there with the understanding that I would give it a global perspective. But when the presidential race got into full swing, it got a lot more domestic than I had signed on for, so this is the perfect outcome," she says.

When asked how her CNN show will differ from the other news talkshows that fill the evening schedules, she hesitates before replying: "Look, I'm not so confident in myself that I think I'm going to reinvent the wheel here. But I think there is room to do new things. What I hope to do is go behind the headlines."

In truth, every news anchor claims that. Although she may not like to admit it, what ultimately differentiates these shows from one another is not the news coverage but the distinctive personality of the anchors and it is here that Amanpour holds a trump card, and why she is so valued.

"She's a woman, she's gutsy and she has an accent," as a 1996 Newsweek profile put it, pretty reductively. More than that, as with the best US cable news anchors – Rachel Maddow on the left, Anderson Cooper in the centre and Bill O'Reilly on the right – there is no sense of artifice, and viewers trust her. She also occupies a rare niche of being unashamedly highbrow, without any of the larkiness to which Maddow and Cooper are prone. Not that this has always been to her advantage, particularly working on US networks: "'American viewers don't care' - How many times have I heard that? About anything serious, anything foreign: 'Oh, the American people don't care.' I have rejected that all my career," she says forcefully.

Her international background, both personally (she grew up and was educated in Iran, the UK and the US) and professionally (her coverage of the Bosnian war and interviews with leaders such as Yasser Arafat remain highlights in CNN's history), gives her a different perspective; and the sheer silliness that characterises so much of American political coverage is the antithesis to Amanpour's approach.

She found coverage of the Republican primaries perplexing during her time on This Week. "So much time was spent giving serious currency to people like Michele Bachmann and Donald Trump when to me it seemed obvious that Mitt Romney was always going to be the nominee. I never understood that," she says.

Yet the network she works for is partly to blame. While CNN remains determinedly neutral – Amanpour prefers "objective" – the other 24-hour news channels offer precisely the kind of gossip and blatant political partisanship she abhors: "Fox didn't take viewers away from CNN, they took viewers who were dissatisfied with all news networks and they nurtured a political base. That's what they do and they shouldn't pretend to be anything else. The trouble is they then exerted pressure on other cable stations to take on more of a political role."

Amanpour herself has been criticised for what some saw as a bias in her reporting from the former Yugoslavia but she draws a distinction between being "passionate" – "I'm not neutral between victim and aggressor" – and "acting like an advocate for a political party, which is not what I do".

She does, however, have close personal ties to some high-profile Democratic figures. As well as being married to James Rubin, the state department's chief spokesman during Bill Clinton's presidency, her college housemate and lifelong best friend was the late John Kennedy Jr and she says Jackie Kennedy Onassis was "a real mentor to me". A photo in her office shows her with Nancy Pelosi, the former speaker (now minority leader) in the House of Representatives, of whom she is clearly very fond.

"I've been very lucky in my career, being a woman in a man's world," she says, "but there's still so much prejudice out there, the old boys' club. Nancy Pelosi said to me, it's taken 200 years for 17% of Congress to be female in a country whose population is 50.8% female.

"At this rate it's going to take another 600 years to get parity in Congress! We need to pay a lot of attention to women's participation in America and Europe as well as Afghanistan and Saudi Arabia and all the usual suspects. And could we please have a female head of a broadcast or cable news network in America already? Come on," she says, all seriousness, all passion. "It is time. Now."