I always felt that James Stewart and Arthur O'Connell had great chemistry in Anatomy of a Murder. O'Connell played an older mentor figure to Stewart whose dependence on alcohol has made him self-doubting and unambitious. Stewart walks a fine line between being an enabler and an interventionist and his studied unconcern when dealing with O'Connell lets us see how deeply he actually is concerned about the welfare of the older man. Stewart sees that an opportunity for himself (a big murder case) would also be an opportunity for O'Connell to turn his life around, so he enlists his help with the proviso that he be sober throughout the process. Stewart actually becomes the mentor, without ever taking any credit for it. It's a beautifully drawn relationship and it's a big part of what makes me return to the film again and again.
Arthur O'Connell also had great chemistry with Don Murray in "Bus Stop".
When he leaves Don Murray at the end - to his own devices and Marilyn Monroe - it is a genuinely sad exit.
Because Murray - without O'Connell and with Monroe - is not a promising prospect.