The line Jimmy Stewart hated to say
From what I've read, Jimmy Stewart was not pleased with his work as Rupert Cadell and, indeed, the film ROPE, itself. I imagine the actor felt stifled portraying an old and tired retired college professor, who espouses Nietzchean "superman" theories, and who, when on his best behavior, is patronizing and acerbic, while in his worst musings is overbearing, insulting and anti-humanity. Coupled with the gay subtext of the plot that touches his character, I bet at some point Stewart must have been thinking how did he ever say yes to this off-beat, subversive little murder mystery. Be that as it may, and even with all that dark character baggage, my feeling is that most of his distaste for his role came down to one line of dialogue, and it's a very particular line that would probably never be spoken by another Hollywood leading man of his day (or since).
The line is, "Good old Mrs. Wilson, I may marry her." Why this line? Well, look who he's talking about, the old biddy housekeeper who's held a torch for him for years. Rupert Cadell is supposed to be similarly smitten with the myopic 50-ish year old character who's more suited to be the paramour of a Walter Brennan or Thomas Mitchell. Stewart puts as much focus on eating his ice cream as delivering this line, which he does as a throwaway, giving more passion to the dialogue, "I'll have another cup of coffee." Stewart does not want the audience to think for a second that he would seriously considering marrying the busybody maid despite the script inferring just that in previous scenes. If not for that, the line would come off as a patronizing bit of whimsy, but Mrs. Wilson and Brandon have made it clear there are deep feelings between Rupert and Mrs. Wilson. Stewart must have blanched when he learned matronly Edith Evanson was cast in the role; and therefore he delivered the line accordingly (like he doesn't mean what he's saying).
One wonders what this does to a star's self-esteem, one who had in previous films romanced the likes of Jean Arthur, Kate Hepburn and Donna Reed, and in a few years hence, trade in those Hollywood beauties for Doris Day, Grace Kelly and Kim Novak. Superstar actor James Stewart seems totally wrong for the pro-authoritarian milquetoast Rupert Cadell, a role better suited for Vincent Price, and Stewart knows it. That is, until the last 1/8th of the film.
The last reel is where Stewart gets to do what he was paid to do. He goes to town in his final speech; the rage, the self-flagellation, the disgust and remorse he displays he does like no one else of his era could do. He pulls the film from its moorings, and gives a seven-minute star turn in bravura larger-than-life acting that Bogart, Cagney and all the others could envy. Stewart the actor must have needed lot of patience waiting for his big moment in ROPE, even allowing himself to say one of the most distasteful lines of dialogue in his long career.