Strong connections to 2 other movies
for different reasons -- The Prisoner of Second Avenue and When Harry Met Sally.
I only saw the Apartment for the first time the other day and immediately thought how Lemmon's Prisoner of Second Avenue is almost a (really depressing) sequel. Virtually the same character, working in essentially the same building, only now it's a decade and a half later. Instead of a young guy with a bright future ahead of him in the City that Never Sleeps he's the beaten-down has-been in the city that's about to be told to Drop Dead. His nascent love affair with Shirley MacLaine has turned into a sniping marriage with Suzanne Pleschette. It was eerie, to the point I kept feeling sorry for CC as though I already knew how his life would turn out.
Then there's When Harry Met Sally. Perhaps everybody's already noticed this or I'm just completely wrong, but in watching the Apartment I could see how Rob Reiner clearly was paying homage to that film in When Harry Met Sally. Especially the whole New Year's in NYC thing at the end. Also, though, there were lots of small things about not finding your soulmate (while standing next to your soulmate) and even the way the front stoop of CC's place looked almost the same as where Bruno Kirby and Carrie Fisher lived.
Funny how movies can interrelate.