The pair star in biopic 'The Imitation Game', in which Cumberbatch plays trailblazing mathematician Alan Turing, who helped break the Enigma code during during World War II. Knightley, 29, plays close friend Joan Clarke, who later became engaged to him, reported Harper's Bazaar.
"Benedict is a very good dinner companion: always interested and interesting, up on politics and the news (so he's got an opinion) and always ready with a story, although occasionally you have to pin him down to one subject because he tends to veer off in all directions," Knightley said.
The actress also spoke about the 'Sherlock' star's meteoric rise to fame, adding: "He is really enjoying his success. He is 38, and has wanted this for such a long time and has worked so hard.
"More importantly, his love for acting is something I haven't seen in anyone else. He has found the one thing in life he is singularly passionate about and he is making the most of it.