It occurs rather early in the film when Cary Grant returns to the Plaza Hotel - accompanied by the fabulous Jessie Royce Landis, playing his glib, eye-rolling mother - to search the room of one George Kaplan, for whom Cary's Roger O. Thornhill, an advertising executive, has been mistaken.
Snooping around in the suite's bathroom, Cary examins a hairbrush.
Walking back into the bedroom, he announces to mother (in a deadpan manner patented by Grant), "Bulletin! George Kaplan has dandruff."
Grant's body language, telegraphing disgust with the idea of dandruff, perfectly complements his line reading. He hunches his shoulders and slightly flails his arms, making spindly movements with his fingers:
"Bulletin! George Kaplan has dandruff."
Brilliant, just brilliant.