The supporting cast of this movie is almost flawless (Mark Lynn-Baker is a little too frantic in some of the more slapstick moments, but VERY good when simply being sarcastically funny or very sincere). With the exception noted above, there isn't a missed beat from a single cast member, and Bologna and Kazan are particularly notable.
But the movie belongs to Peter O'Toole, and, as in The Stunt Man (made a few years earlier), when he's on screen you can barely take your eyes off him. Whether being hysterically funny or terribly sad, he owns the screen from start to finish. From the first shot of him, waking up embracing what he thinks is an unremembered lover from the drunken night before, and staring in bewilderment and near terror as he realizes that it's not a girl at all but a furry THING (a teddy bear), to the last shot of him as he accepts an ovation from the studio audience in probably the only truly serene joy he's had in decades, he is utterly perfect and utterly compelling. Richard Benjamin may have directed well, the script may have been terrific, the supporting performers excellent, but this movie could not have been what it was without O'Toole's presence and brilliance.
It's hard to imagine a more enjoyable way to spend an hour and a half than in watching this movie. There are so many little jokes, so many big jokes, such lovely and REAL touching moments. It is, despite its occasional hilarity, a very gentle and loving movie; there's just no meanness in it, which is a very pleasant and rather unusual thing, especially in a comedy. Well worth watching and re-watching. You'll love it. Your parents and/or grandparents will love it, and you wouldn't be embarrassed to show it to them.