Like many critics, I tend to mean something good when I describe a film as 'powerful'. But like many people, if you asked me if power was always a good thing, I'd answer that it all depends on how it's used.

This semantic contradiction was brought home to me at the end of the Cannes screening of Clint Eastwood's undeniably 'powerful' new film, The Exchange (initially sold as The Changeling). There I was with a tear in my eye and a lump in my throat - and an angry resentment in my mind at the crude way in which my emotions had been coerced.

Let me say straight off that plenty of colleagues whose opinions I value liked The Exchange. But I also talked to a couple of festival-goers who shared my unease. Glad to have found each other, we fuelled each others' iconoclastic resolve with words like 'manipulative' and 'heavy-handed', to the point where (as often happens in these clandestine outpourings) we lost sight of the film's merits - its exceptionally clean and lucid narrative, its feel for period detail, its classic, sombre cinematography and a career-best performance by Angelina Jolie.

Then I remembered another Eastwood film, Mystic River, had provoked a similarly bipolar reaction in me; there too, anger at being used had been my final response to a dark and primal emotional ride.

Nothing a good shrink wouldn't be able to sort out, you're probably thinking. But just for the sake of argument I'm going to assume there's more going on here than my own Anglo-Saxon inability to give way to emotion without intellectual override. After all, films like Casablanca or I Know Where I'm Going! choke me up every time.

I'm convinced the sort of spectator schizophrenia I'm describing here is relatively common - though it's rare for film critics to admit to it in reviews. And I think it often stems from what we feel to be a kind of abuse of power on the part of film-makers.

Some of this has to do with the choice of story. Certain narratives touch deep chords within us - some of them related to society's strongest taboos. It's almost impossible not to be shocked by a tale of a stolen child, as the psychic grip of the Madeleine McCann abduction demonstrates. In returning to the same material twice over (in Mystic River and The Exchange), and in indulging our worst fears in both cases, Eastwood might be accused of confusing moral condemnation ("Isn't this terrible'") with facile audience electro-shock ("I'm going to show you just how terrible this is").

If such potent narratives are accompanied by deterministic plotting that excludes the randomness of the real world; if they're supported by a mix of big religious themes (redemption, guilt, good and evil, the lasting effects of violence) and Western values (stubborn individuals, the wandering loner, taking justice into one's own hands); if they're channelled through characters that are archetypes (Mother Courage, the evil cop) disguised as real people - then the film will inevitably be 'powerful', especially if some of the best actors working today are hired to smooth down its lurid edges.

Emotionally empowering

At the root of this is the idea that it is relatively easy for a film to arouse the emotions, but less easy for it to do so in a way that retains the audience's respect. There was a point in The Exchange, around two-thirds of the way through, when I felt so pummelled into submission that I decided to fight back - even though my emotional responses kept running on autopilot.

Possibly, in another mood, or with more breakfast inside me, I might have let myself be pummelled and come out of the screening feeling impressed but vaguely used.

Actually, it was another film - Laurent Cantet's deserved Palme d'Or winner The Class - that helped me to strengthen my case against The Exchange. There's no emotional coercion in this French junior high school docu-drama, no narrative baseball bats; in fact there's very little story at all for the first hour or so - just peppy, engaging classroom debate and badinage.

And yet The Class turns out to be just as 'powerful', in its own way, as Eastwood's big, heart-stopping, tear-extracting fairground ride of a film. The difference, for me, was that I came out of it feeling intellectually and emotionally empowered, rather than passive and powerless.