When I first saw the movie, I was familiar with Cary Grant as the elegant, suave protagonist of movies like To Catch a Thief. I hadn't realized, until I saw Arsenic and Old Lace, how much comedy Cary Grant brought to his parts. It was always there, I just hadn't appreciated it before.
I'm currently reading a biography of Grant by Marc Eliot. I've learned that Grant's background was . . . vaudeville would be the best word. He was part of an acrobatic troupe when he came to New York. But in his first years in Hollywood, he was mostly cast as the "elegant guy in a tux," a role that demanded little from him and kept him in the second tier of acting. (He kept getting roles that Cooper discarded.)
And then Grant broke from the Academy and become an independent contractor, an incredible brave move in those days and also, the reason the Academy refused to give Grant any awards until 1970 when Gregory Peck insisted that they honor Grant with a lifetime achievement award. (I really don't know how anyone can believe that the Academy awards aren't anything but political grandstanding.)
As an independent contractor, Grant got lucky. He got cast in The Awful Truth where his comedic powers made the picture the hit of the year. And then Hitchcock discovered him and developed Grant's darker side. What Grant excels at IS elegant suaveness but with a soupcon of black comedy at the back of it all. It's the snaky part of his smile. In his biography, Eliot bemoans that Hitchcock couldn't get Grant for Shadow of a Doubt. However, I think Joseph Cotten was the better choice. The psycho uncle in Shadow of a Doubt has to disgust the viewer. The excellent script by Thornton Wilder emphasizes that this is a good town and a good family ("decent"--high praise from Wilder) that doesn't deserve this particular problem. The audience has to want Cotten to leave, has to feel more and more panicky as he continues to stay. The problem with Cary Grant is that even when he is scary, you want him to stick around. Oh, so, he might kill a few people, what's the prob?
But he is perfect both in Suspicion and Notorious, where his dark side really shines (of course, Claude Rains, at 5'6", outplays everyone in the latter, with the exception of Bergman; Notorious is a truly great film). But to return to comedy, regarding The Awful Truth, Eliot writes:
Grant's catlike physicality, which had brought him to the brink of lugubriousness in his earlier leading-heel roles, now translated into a youthful, rhythmic prance fueled by the high energy of light comedy. A bend of his knee become the equivalent of a punch line. A lifting of his palms expressed a lifetime's skepticism. A tilting of his head suggested a turning of the other cheek (page 164).And I think that Eliot's insight here about Grant's physical (vaudeville trained?) comedy is right on. It is this plus Grant's dark comedic side that explains (or helps explain) his star quality; the amazing Cary Grant who makes it all look so effortless.
On a side note, one thing about the Hollywood of that era is that it was far more drug-ridden, debauched, political and just crazy than it is now, we just know more about it now, what with Entertainment Tonight and such. But the backstabbing, backbiting of "The Golden Era" was truly astonishing. And boy, didn't everyone dislike Katherine Hepburn! Now, there's someone who MADE Hollywood love her.