From Robert Rose…
The subject of It's a Mad, Mad, Mad, Mad World came up in a Marx Brothers discussion group I participate in. Opinions on the movie varied (the most interesting to me was the fellow who says he loves the film but that it isn't funny; I'm not sure how that works) but of course in that group the burning question was "Why wasn't Groucho in it?" There were various explanations and theories advanced, but rather than list them I thought I'd toss the question your way, as an acknowledged expert on the film who also seems to have come across the occasional stray fact about the Marx Brothers, too.
(I did first try to search through your blog to see if you'd addressed the question before, but eventually gave up, defeated by the sheer number of posts that mention Groucho, some of which even do so without mentioning Frank Ferrante.)
I hope people understand that even an "acknowledged expert on the film" can't know everything for certain and that if you'd asked Stanley Kramer at the time about the movie, there are some things even he wouldn't know. A movie involves a thousand decisions and you can't track and dissect every one of them.
I can tell you that a lot of the casting decisions were last minute "get anyone you can" decisions. On a film like that with so many set-ups and so many players and so many stunts and tech problems, you can never be sure when you'll get to certain scenes. A lot of folks who were not in the film weren't in it because they were shooting another movie or a TV series at the time.
I asked Dick Van Dyke and he said he was never approached. (Carl Reiner was in it but since Carl wasn't in every episode of The Dick Van Dyke Show, he did a lot of other things while that series was in production. Morey Amsterdam recorded a voiceover for Mad World that was cut from the film but that was a voiceover that might have taken an hour.) Anyway, that's probably why people like Lucille Ball, Danny Thomas and Jackie Gleason weren't in it. Joe Besser was cast as one of the gas station attendants but couldn't get time off from The Joey Bishop Show.
Stan Freberg walked onto the set one day to discuss the commercials he was producing for the film. When Kramer had a break in filming, he instead said to Stan, "What size shirt do you wear?" They needed someone to fill a role they were prepping to shoot an hour or so later.
Howard Morris is nowhere in the film, nor did he ever appear before the cameras. But on two separate occasions, they booked him for a day's work (and paid him) because they thought they might need him to replace someone else, depending on scheduling. He never knew what the role was and the following is just my theory…
I suspect he was engaged in case the schedules on Mad World and The Andy Griffith Show didn't come into proper alignment so Don Knotts was available. If when they needed to shoot the scenes with Don, he couldn't get away from Mayberry, Howie would have been in that part and people would be asking, "Why wasn't Don Knotts in the film?" Or maybe Knotts would have wound up in a different role.
Either of those would have been a shame because he was so perfect in the part he played. Don made such a strong impression in the movie that people think he was in a lot more of it than he was. His total on-screen time totaled exactly 120 seconds.
Mr. Kramer told me that Sterling Holloway's casting as the fireman was one of those "we couldn't find anyone else when we needed someone" moments and that he hoped there'd be an opportunity to reshoot it later with a bigger star — and there wasn't. I theorize the part was written for Ed Wynn…and hey, that might have been a good spot for Groucho. I've heard there do exist script pages for a never-filmed scene where Groucho would have cameoed as a doctor treating all the major male cast members at the end.
It would have given Groucho the last line of the movie and they might have decided that at that point, the focus of the last scene should have been on the stars of the film instead of on an interloper. Or Groucho might not have been available. Or Groucho might have wanted too much money. Or there might have been some reason no one could have imagined.
At some point during the filming, a small role was apparently offered to Jack Carter. Mr. Carter was fiercely competitive with other comedians and he wasn't about to come in and do two lines in a movie starring guys like Milton Berle and Sid Caesar with whom he had what we might call "rivalry issues." He turned the job down saying something like, "Call me when you have a real part." They never did, either because one did not come along or because they just decided not to deal with someone who took that attitude. The one time I got to talk to Stanley Kramer, I asked him about it and he said, "I don't remember. I probably had too many things to think about." That's the way it is with some things.