Until about 20 minutes before it finished, "Spring Fever" was a disappointment and William Haines, the leading man, was really getting on my nerves. He was lacking that something special that made him so engaging in King Vidor's "Show People" the only other film of his I have seen. His character was arrogant and conceited and rather uninteresting really. As far as I found out, this was very much his screen persona. There were also some private jokes (possibly not that private) about his sexuality, including one in the very beginning which surely could not have gone unnoticed (it's mentioned in the IMDb forums for the film, if you're curious). However, I believe a comedy should be funny on its own merits and not because it winks at the audience or its camp value. There was also too much pantomime, not enough acting and certainly not enough wit.
The film is clearly a factory product to sell William Haines. Not a single risk is taken, not a tiny bit of imagination is allowed. It worked before, it will work again, seems to have been MGM's motto for this production. This affected the supporting cast. Lead by a rising Joan Crawford, all give quite good performances but none is on-screen long enough.
So what changed in the last 20 minutes? In fairness, not much. It's still predictable, still formulaic. It has a minor twist which even if it comes as a surprise, it will be a mild one. The difference is that it has charm. Joan Crawford has now a few key scenes and that counteracts Haines' excesses. And they act together. It's a joint effort, not a star-vehicle. Suddenly I stopped seeing the gay guy and his fag-hag (Haines and Crawford were very good friends off-screen) and actually saw a romantic couple. And for those last few scenes the film clicked into place. If you want proof, look at the titles in the hotel room - they're actually funny and witty.