The common wisdom concerning the penultimate scene of Psycho (1960) has always been that the speech delivered by actor Simon Oakland - in which his psychiatrist character explains in exacting detail why Norman Bates has been committing murder while dressed as his dead mother - is a tedious attempt at summation as Oakland is given the thankless task of walking us through Norman's twisted mind, dryly explaining the craziness we've just witnessed. But while this scene is usually singled out as a misstep, a speech that could've used some judicious editing and still conveyed the necessary info, I feel like there was an underlying method to Hitchcock's madness.
Every time I've watched Psycho, I've always felt that Hitchcock wanted this scene with the psychiatrist to work on two levels. One, I think he felt that a large part of the audience would really need an explanation and that he was obliged to include this scene for the sake of clarity. Even though what we see transpire in the fruit cellar is enough to roughly put it all together, a more deliberate connecting of the dots had to be there. But I also feel that while Hitchcock knew he had to include that scene, he purposely portrayed the psychiatrist as a windbag - knowing that he would let the air out of everything that was said with the coda that followed with Norman alone in his cell. Oakland plays the psychiatrist as a self-satisfied blowhard who likes the sound of his own voice. He's smug, he's comfortable playing to an audience. After talking to Norman - or specifically, to Mother - he's got the whole story. His explanation is all about demystifying what we've just seen. He takes all the mystery out of it.
But then Hitchcock pulls the rug from under that speech by bringing us back to Norman and letting us hear his thoughts as Mother. While everything that the psychiatrist says about Norman - about his crimes, about his split personality - may be true, the last scene with Norman shows just how empty those words are. Hitchcock could've let the audience off the hook with Oakland's explanation and left the film at that. That would've been the conventional choice. Vera Miles and John Gavin could've walked out of the police station with matching sad faces as soon as Oakland finished talking with a big 'The End' title imposed over them - Janet Leigh may be gone but hey, at least normality is restored. But for Hitchcock to go back to Norman instead and let Mother's thoughts be the film's final words (courtesy of actress Virgina Gregg) is a brilliant undercutting of Oakland's speech. By doing this, Hitchcock is able to have his cake and eat it too. Yes, he gives the audience the explanation but then he shows how bullshit it is to believe we can understand a person as disturbed as Norman.
What's always made my skin crawl the most about Psycho was imagining what Norman's victims saw in the last moments of their lives. To know that these people suffered a death that was inexplicable to them - to see who was attacking them, to be able to recognize Norman (even though in the shower scene we only see Mother in silhouette, I always felt that Marion could see Norman's face just as well as Arbogast clearly does) but to have no way of comprehending why Norman was dressed the way he was or why he was out to slaughter them - was an idea that burrowed into my brain. And when Hitchcock returns to Norman after the psychiatrist has had his say, he is putting a fine point on the idea we are eternally vulnerable to the madness of others. This is what Hitchcock wants to leave us with, not Oakland's hollow explanation. The psychiatrist can dissemble Norman's mental state with practiced professional acumen now that Norman is in custody but the truth is, if this psychiatrist had gone to the Bates Motel a day earlier, he would've stood face to face with Norman and not perceived his insanity.
By knee-capping the psychiatrist's speech, Hitchcock obliterates any comfort those words might've offered, allowing Psycho to endure as the ultimate public service announcement for watching your ass at all times.