This script never gets the credit it deserves. Most of the acclaim to this masterpiece deservedly goes to Hitchcock, with leftover claps to people like the set designer, Tony Perkins, Saul Bass, Bernard Herrmann, Janet Leigh etc. (not even die hard fans are too enthusiastic about John Gavin and Vera Miles).
But it really is superlative work from Stefano - I've no doubt that Hitchcock rode shotgun on this, but Stefano does a great job, introducing a vivid character in Marion Crane, and her desperate love for Sam Loomis. The embezzlement sequence is fantastic, full of tension and excitement even on the page with some clever lines eg the secretary assuming the businessman didn't look at her because she had a wedding ring. The exchanges with Norman are a bit 50s psychology at times but he's such a wonderful character it doesn't matter.
Then the action switches to Norman - Act Two centers around the investigator Arbogast, Act Three focuses on Sam and Marion's sister. There's some odd repetition in Sam telling the whole story to the sheriff.
William Goldman always calls the scene at the end where the shrink explains everything a snooze scene - but personally I like it, it's nice to have things explained, and the movie does recover for a creepy look at Norman.