A brilliant portrait of a formidable woman.

Two decades in the capital, and I'm still not used to the middle-class double kiss, especially when, as is increasingly the case, it is bestowed on me by a person I barely know. So it's not difficult to imagine how I would have felt about Mo Mowlam, hugger and kisser extraordinaire, had I met her in real life. The horror. The creepy, manipulative incontinence of it. In her presence, I would have been cast in a David Trimble role: buttocks clenched, lips pressed tightly together, raincoat in hand, the faster to escape the woman's looming forearms. In meetings, we gather, Trimble refused to call her "Mo". For him, the words "secretary of state", icily spoken, were his best defence - a kind of verbal garlic - against Mowlam's vampiric need to bury her face in his neck.

In Mo (31 January, 9pm), Neil McKay's brilliantly written film, Mowlam's hugging was deployed to great effect. The more she hugged, the more suspicious you felt of her chumminess, of her supposedly unique ability to "connect" with other people, be they loyalist terrorists, teenagers with learning difficulties or the president of the United States ("Gerry Adams is so in awe of you, he'd go camping up your arse if you asked him to," she said to Bill Clinton, on the line from Washington).

All that "warmth" and physical contact started to seem like a kind of bullying. Mowlam was, literally, a woman from whom it was impossible to escape. In one scene, she followed her junior minister Adam Ingram (Gary Lewis) into the gents, standing by the urinal as he took a leak ("Hmm, not bad!" she said, gazing down at his manhood). I'm not inclined to look indulgently on those who refuse to let their fellow human beings relieve themselves in peace.

So, brilliant writing. But also, great casting. Yes, Adrian Dunbar's Trimble and John Lynch's Adams made me titter just a little. (It's naughty, I know, but I find Northern Irish politicians fundamentally preposterous.) But Julie Walters turned in a perfectly miraculous impersonation of Mowlam: her fluting, prissy voice; her half-closed eyes and half-open mouth; her cool, calculated intensity. Walters, for all that she is so lovable, has the beadiest eyes and, in the middle of a hug, a rude joke or a good scratch of her wigless scalp, you could see them working, gauging the reactions to her performance, much as I imagine Mowlam's used to.

McKay, who put in months of research before embarking on his script, delivered a bona fide scoop in Mo. Thanks to her doctor, Mark Glaser (played by Toby Jones in the film), he learned that she consistently lied about the seriousness of her condition. But it was also, more subtly, McKay's contention that her tumour and her personality grew increasingly indivisible (brain tumours can cause disinhi­bition), and thus that it was the latter as much as the former that helped lead to the signing of the Good Friday Agreement.

Can this be true? I'm not qualified to say, but if her tumour did intensify her touchy-feely ways, her fondness for sex and vulgar jokes, then it surely would have had the same effect on her other character traits. In the film, Mow­lam's husband, Jon Norton (David Haig), was portrayed as a needy, sometimes buffoonish man, one whose inability to accept the gravity of his wife's illness found a perfect form of denial in encouraging her to believe she might replace Tony Blair as Labour leader. And Mo - or her tumour, or both - responded: inside, the dragon of blind ambition flicked its tail.

We find ourselves in the middle of a deluge of biopics right now. This is the third time in less than two years that Walters has starred in one (she has also played Mary Whitehouse and Anne Turner, a terminally ill doctor who died at Dignitas). The contaminating effect of reality television is that even our dramas must be “real" these days. But McKay brought something extra to Mo. He brought a genuine spirit of inquiry. In a small way, he may even have helped to rewrite history. I don't think I would have liked Mowlam. But I liked this film very much for its refusal to sentimentalise, and for its grasping of the difficult point that the line between a person and their illness can often be fuzzy in the extreme.

Rachel Cooke trained as a reporter on The Sunday Times. She is now a writer at The Observer. In the 2006 British Press Awards, she was named Interviewer of the Year.

This article first appeared in the 08 February 2010 issue of the New Statesman, Nightmare on Cameron Street