CHARADE: Cary's lost his sparkle

Much has been written in the decades since it happened about Cary Grant's decision to retire from acting in 1966. Despite his assertion that he was too old to continue playing the leading man convincingly many of his fans insist Grant could have gone on romancing ladies half his age for many more years to come.
On the evidence of CHARADE I believe Grant made the right decision.

He was all of 59 when this comedy-thriller was released in 1963 which is certainly not old, but he looks tired. The sparkle that made Cary Grant 'Cary Grant' is missing and he plods through the film with the air of a man who's getting "too old for this shit." There's no sense of enjoyment in his performance even though Audrey Hepburn's character makes it quite clear to him that she's his for the taking.
While in real life Grant was attracted to younger women he apparently felt it unseemly to be seen pursuing ladies young enough to be his daughter on screen and perhaps he allowed this discomfort to influence his performance, even though the script had been changed to make him the pursued party.
Whatever the real reason the result is disappointing. CHARADE never comes to life in the way that "To Catch a Thief" or "North by Northwest" did a few years earlier, The blame is not entirely Grant's. While Walter Matthau is impressively understated, George Kennedy and James Coburn play their parts too broadly to be convincing heavies and Hepburn is (ironically) starting to look a little too grown up to be entirely plausible as the naif.
Grant's talent, even when not firing on all cylinders, was considerably superior to may other lesser stars on their best day, so CHARADE is nothing to be embarrassed about and the film did respectable business at the box office on its original release.Grant though, was reportedly stung by the reviews which focused more on the age gap than the acting and he would make just two more movies before calling it quits.