He would still have been a great director, the consummate innovator and craftsman, but not quite as driven by a need to dominate, torture and remake women. For Hitchcock, fat was his curse and his gift. Fat made him self-conscious, lonely and inward as a child; it also made him funny, ruthless and honest as a man, with a taste for make-believe cruelty.

Fat made him look at women with hungry eyes; he knew they would never see him, but he saw them. That desire drove his pictures like a piston. Fat was his friend, at least in a creative sense.

Hitchcock made light of his appearance, but he hated being overweight. He satirised himself with a chubby-cheeked logo that became his brand, recognised the world over. No other director has done that. In effect, he turned his misfortune into a trademark. He also turned his fears into assets, his obsessions into the engines of his stories.

Much has been written about his taste for cool blondes. In the new movie Hitchcock, Anthony Hopkins plays the director in 1959, making Psycho. We see him drooling in private over photos of blonde actresses, something that disgusts his wife Alma (Helen Mirren). That may be how he felt, but it misses the point: he chose blondes for what they represented to an audience, as much as for what they represented to him.

He chose the most elegant archetypes: Grace Kelly, Eva Marie Saint, Ingrid Bergman, Kim Novak, Tippi Hedren. They were the unattainables, the mannequins, the ladylike creatures he could bend to his will in roles that often humiliated them. He didn't like vulgar ''sexiness''; he liked a certain innocence that he could vulgarise, for the audience's entertainment. That's why the shower scene exists - Janet Leigh's beauty is violated, her ''innocence'' corrupted. He got even with the beauty, on behalf of all the lonely fat guys out there who could never be with her, and the women who could never look like her. Everyone was pleased. We were all like ''Mother'' upstairs, willing Norman to do his duty.

Of all the prominent directors, Hitchcock is probably the most commercially minded. He learned one important lesson as the son of an East End grocer - give the public what it wants and make it fresh, whether it's broccoli or blood.

At the same time, he came into movies in 1921 equipped with a ruthless honesty, a gift for analysis of himself and humanity he took from a Jesuit education. Most of his pictures do not flatter the watcher: they show us something horrible within ourselves. Virtue is punished, as much as sin. Why did the birds attack poor Hedren? What had she done to deserve that, apart from being spoiled and rich? Leigh stole $40,000 in Psycho, but she was going to give it back when ''Mrs Bates'' struck.

Hitchcock was a very Catholic artist - he knew that we should all be punished, even if we had not yet committed a crime.

He didn't just torture his actors. He told Francois Truffaut that tormenting the audience was the proper role of the director. ''Putting them through it,'' he called it. He also believed in putting himself through it in the process of preparing the movie - the first catharsis would be his.

He remained a Catholic for life, and his films have lots of appearances by nuns and priests - most notably the nun who comes out of the shadows and scares Novak to death at the end of Vertigo.

French auteurs Claude Chabrol and Eric Rohmer studied his films as Catholic texts, identifying some key aspects. One is that moral lapses beget the wrath of God in the form of suspense, delivered by Hitchcock. The other is that guilt is transferable, so that every character is incriminated, along with the audience.

The shower scene broke every rule of the Hollywood scriptbook, by killing the heroine in the first half. It was morally confronting: she was a thief, but no one deserved to die like that. It made us complicit, by turning the violence into spectacle. Most of it was shot from the killer's point of view, so that we also ''held'' the knife. Hitchcock, as God, dispensed retribution and we helped by watching. You are no better than me, he seemed to be saying.

Blondes were always a fetish object for cinema, partly because they photographed well in black and white. It is true that in The Birds, Hitchcock overstepped the boundaries of formal civility he usually set for himself. Hedren never forgave him for the three days of extreme discomfort she endured to get the scene in the attic. And she was probably never better as an actress. ''A beautiful woman is a force for evil,'' he once said, meaning that evil follows beauty, rather than that beauty is evil. A lack of beauty may also be a forceful impetus, as it was for Hitchcock, the chubby genius.

Hitchcock is out on Thursday.