Most people know him, of course, from his role in the film version (and the Broadway play) of THE ODD COUPLE. And then later in life he did some films with Jack Lemmon that were popular (which is probably why I don't particularly like those films). And he did some things on TV toward the end of his career which were not that good; but I reckon you take what you can get when you are an aging actor and choices are limited.
However, he sometimes did movies that I have always figured were against type and which I thoroughly admire, partly because he was indeed cast against type in such films. These movies were also directed by people I quite admire and seem to be rather forgotten these days, but I like to recommend them to folk looking for a good movie to watch.
One of these films is A NEW LEAF. It was written and directed (and co-starred) Elaine May. Matthau portrays Henry Graham, a playboy who has realized that he has burned through his inherited millions and is facing poverty. He talks a hateful uncle into advancing him some money so that he can continue to pretend to be rich for a few more weeks so that he can find and marry a millionairess (the idea of marriage having heretofore been an idea repulsive to him).
Pretty much everyone in the film turns in amazing performances, including James Coco as the vengeful uncle who does extend the loan but who does so only because he thinks Henry will fail. Elaine May cast herself as the heiress he finally lands and she, too, turns in a priceless performance. In no time, Henry (who has no intent to remain married) begins to plot his wife's murder. Will he go through with it, or will some shadow of conscience cross his mind?
It's a great movie. Give it a shot. (In a day or so I'll list another such fine Matthau film that remains obscure.)
"Don't let 'em out!"