Character Analysis
For a main character, Sam Loomis is surprisingly… absent. He just isn't given a whole lot to do onscreen.
But Sam's the trigger for the plot; he's in debt in part because of his ex-wife, and as a result he feels like he can't marry Marion. (He can, however, fly out to see her in Phoenix and have sex during her lunch hour.)
But if Marion marries him, he tells her, she'll have to "live with me in a storeroom behind a hardware store in Fairvale." And Sam is too ashamed of this fact to pop the question.
Marion steals the money to give to him and help him out of his financial difficulties. She doesn't tell him her plans, though. He's not really part of the plot till she dies… and then he's secondary to Arbogast, who takes up the search for Marion. And then he's really secondary to Lila, who's the one who insists they investigate further (and is also the one who gets to do all the fun stuff, like find Mrs. Bates' corpse).
This leaves Sam with what, to audiences in the 1960's, is a pretty stereotypically feminine role. Marion acts; Sam responds. Marion's the breadwinner, and she's going to solve all Sam's problems so she can sweep him off his feet. And that leaves Sam with the job of worrying, fussing, and mostly standing off on the sidelines.
Though he doesn't get stabbed to death. So it could be worse.
Sam Loomis' Timeline