Ron Rich
Of course Matthau, Lemmon and Wilder are great. But I just wanted to add a comment that I thought Ron Rich's performance was good as well (as Luther "Boom Boom" Jackson). I am a little surprised that he didn't do more movies after this. I see that he did a couple of movies and some TV in the 60's and 70's. He was also a producer for one movie in 1987.
I'm curious, does anyone know anything about him? His biography, etc.?