Old guys. They fill their long, fixed-income days and their TV-dinner nights wistfully recalling past potencies. If we are to believe two new movies, they also spend a fair amount of time plotting new conquests -- and not necessarily of age-appropriate ladies either. The rest of their long hours they pass bickering.
Walter Matthau and Jack Lemmon are awfully good at this sort of thing. You might say they've been practicing since they were (relatively speaking) kids. By this time they instinctively know how to bring out the comic best in each other -- Matthau's bullying misanthropy, Lemmon's melancholic good cheer....