Billy Wilder’s genius in «The Apartment» hinges on his casting. For Jeff Sheldrake, the duplicitous corporate boss who plays with Kubelik’s heart, Wilder cast against type by hiring Fred MacMurray, who was best known for family-friendly films like Disney’s «The Shaggy Dog» and his role as the friendly father at the center of the TV sitcom, «My Three Sons.» MacMurray’s clean-cut, paternal looks make him all the more despicable as he strings the lovelorn Kubelik along.
The part of Baxter, on the other hand, had to be played by a likable actor. The audience must believe that Baxter is kind and trustworthy enough to care for MacLaine’s Kubelik while she is passed out in his bed after a suicide attempt. Audiences are able to see themselves in Jack Lemmon’s character; you’re convinced that he really is a poor schlub who brown-noses his colleagues and gets shut out of his own home.
In the same commentary, Lemmon’s costar, Hope Holiday, accurately describes the actor’s teddy-bear-like qualities as «cute, cuddly, and warm; you loved him.» It’s a sentiment that was echoed by his colleagues in Hollywood, who dubbed him «the nicest actor we’ve had on the lot» (via The Guardian).