When they adapted the 1944 film from the Broadway play, they brought along Josephine Hull and Jean Adair to recreate their original roles as the sweet old sisters, and also planned to use the original star from Broadway who plays the role of the murderous brother, whose always driven to a rage whenever anyone says he looks like Boris Karloff, an infuriating occurrence for him since the plastic surgeon who worked on his face to disguise his appearance from the police had been drunk when operating and had just watched a Boris Karloff movie. The actor who played that role on Broadway was...Boris Karloff!
As I said, the filmmakers quite obviously wanted Karloff was play the role in the film (that starred Cary Grant as his brother). The problem was that the move was to be shot early in the play's run on Broadway -- though not to released until after it closed, by agreement with the show's producers. (Given that it was a huge hit, running for 1,444 performances, that ended up being three years later in 1944.) The actresses who played the two sisters, Hull and Adair, were given an eight-week release in 1941 to go to Hollywood, but the producers wouldn't also let Karloff go -- not just because that would mean losing three of the leads, but also the show's biggest name. And so Boris Karloff playing a killer who's furious because he looks like Boris Karloff was lost to movie history.
(When Frank Capra directed the film, he wanted the replacement actor, Raymond Massey, to be made up to look like Karloff. Studio lawyers were concerned that Karloff might sue over the use of his likeness, so they asked him to sign a release. As it happens, he was happy to sign -- since he was an investor in the stage show, and it was to his benefit.)
Though it was a huge lose to film history not having Boris Karloff in the movie, he did happily get to play the role again for national audiences, and twice.
The first of those times was in 1954. NBC did an hour-long radio adaptation of the show, and Karloff got the chance to finally play 'Jonathan Brewster' in a national production. It's hardly the same thing, of course, since a large part of the fun (and point) is that the character looks like Boris Karloff, and this is radio. But he also sounds like Boris Karloff, of course, so at least it works on that level. And great to have had it done. Also fun is that Jean Adair recreates her Broadway and Hollywood role as Aunt Martha Brewster.
Then, in 1962, the show was adapted for the Hallmark Hall of Fame. And Karloff finally got the chance to recreate his role on film. Tony Randall starred in the production as Mortimer (the Cary Grant role in the film), and in a smaller role Tom Bosley played Uncle Teddy (the crazy relative who thinks he's Theodore Roosevelt). Alas, I haven't been able to find a video of this production -- yet. I do keep reading that it's out there somewhere. And being a Hallmark Hall of Fame production that means it has the possibility of showing up at some point.
But the radio broadcast does exist. And here it is below. I haven't listened to the whole thing, but what I've heard is great fun. If you specifically want to hear Boris Karloff -- finally playing "himself" -- he comes in around the 21-minute mark.