Walken started off as a song-and-dance man, actually. DeNiro’s comedy is almost entirely based on self-parody of his image as a serious dramatic actor. The same, to a lesser extent, is true with Neilsen. And while I’m tempted to argue that Grier did not start as a serious actor, it’s rather a moot point, because he’s never been any good at comedy. His career largely exists because the Waynes brothers keep casting him in their movies to make themselves look funny by comparison.
Leslie Nielsen was the type of “B” movie actor who always got cast in supporting roles in disaster movies- for example, he was the doomed captain in “The Poseidon Adventure.” Like Robert Stack, Peter Graves and Lloyd Bridges, he’d spent years playing such roles SERIOUSLY.
When the Zucker brothers wanted to spoof disaster movies in “Airplane!,” their genius was to fill the cast with EXACTLY the type of actors who’d really be hired to play similar roles in a legitimate Irwin Allen-style disaster flick.
Leslie Nielsen was hilarious in “Airplane!” less because he’s a gifted comic, but because he was able to play a comical role as if it were a serious role. He had to say idiotic things as if they made perfect sense.
Actually, that’s a bit sideways. I heard Nielsen say in an interview that he always found he ‘serious’ roles faintly ridiculous and that he played them like the straight man in a comedy. He swore he played Airplane just like every other movie up to the time.