Walker died in 1951, aged 32, only a few months after the film opened. The death was untidy: for several years, the actor had been unstable and a drinker. He was given drugs to reduce one more emotional outburst, and he never regained consciousness. Was the dose wrong, or had Walker brought it all on himself in some suicidal mood? No one really knows. But it was said that he had never recovered from the divorce from his wife, Phyllis.
They had married, when young actors, in 1939. They had two sons. But then, as Walker began to make his way as a naive romantic lead, Phyllis was discovered by producer David O Selznick. He changed her name to Jennifer Jones, made a star of her, and let his first marriage collapse so that he could marry her.
There was nothing Walker could do stop it; no matter that he was close to a star himself - with Judy Garland in the film, The Clock. He broke down. He married again, and that union failed. He was in an institution for several months. And he came back "better" and laughing, but in ways that alarmed his friends.
That's when Hitchcock noticed him. Now, it's easy for us to conclude that Hitch was always a success. Not so. In l950, he had had four flops in a row - The Paradine Case, Rope, Under Capricorn and Stage Fright - and he was anxious to put an end to that run. He had a novel by Patricia Highsmith that contained this superb idea: two men, strangers Bruno and Guy, meet on a train and are drawn together discussing their vexing kin. A plot is hatched, and Walker's eyes come alive with the sheer intellectual excitement of the idea - they have the kind of shine that the eyes of physicists on the Manhattan Project gave off when the beauty of certain mathematical equations was realised.
Guy laughs this passionate idea away: he thinks Bruno is weird. But Bruno's so much more than that. He's inspired by craziness, and he believes that a contract has been entered into. And then, in one of Hitch's finest sustained sequences, with the sinister married to the comedy - since we long for the disagreeable wife of Guy to be dispatched - he has Bruno track the female victim to a fairground, pursue her, half charm her, and then strangle her. Gently he lowers the corpse into our lap, as the prize we have earned.
This sequence was vital in Hitchcock's progress. Never before had he so grasped the way action on screen could be the expression of the audience's voyeuristic desires. Never again would he lose that barbed innuendo - it is the vital mechanism in Rear Window, To Catch a Thief, Vertigo, North by Northwest and Psycho, and depends upon the unconventional insight that we may like a villainous character sufficiently for the achievement of a stealthy ambivalence. And it was Walker who helped Hitchcock to learn this lesson. For his killer is so much more beguiling and compelling than Guy, the stooge, who is played by Farley Granger.
I wonder how much of this the actor and director understood, or discussed, at the time. Walker had always been the soul of kindness, sweetness even. He had a high, hushed voice which Hitch turned into eloquence. The seed lay in Highsmith's searching novel. And Walker was well aware of how much Hitch had done for him.
The film's opening conversation scene on the train is like a tennis match in which Guy serves up one weak lob after another for Bruno to put away. I suspect the things we might see so plainly now - the homosexual longing in Bruno, the wicked contrast of the man of action and the man of ideas, the superficiality of Guy and the hungry depth in Bruno - were never mentioned.
For Bruno was so far ahead of his time. No censor jumped up and said the guy's a fruit - as equally, in l960, on the release of Psycho, no one saw how subversive a figure was its central character, Norman Bates. Everyone settled for Bates being a killer; as if that got anywhere near the real intelligence, the terrible charm of him, or Bruno.
So it is a landmark performance. You see it now, and feel the vibrancy of the modernity. But back in l95l, Bruno was politely ignored.
What might have happened to Walker had he not died then? It's not that movies were well-stocked with parts like Bruno. And Walker was putting on weight. He might have taken up the silky load of master villain actor Sydney Greenstreet, who died in l954. Yet he was likely headed for some bad ending. Even so, he had had that one chance - so that, decades later, we just can't get him out of our heads.