The tension in Roman and Logan’s scenes is palpable, but it’s not strenuous acting work, according to Kieran Culkin. He told NPR:
“Just being in a scene with someone like Brian, there’s a lot less work for me to have to do. … Brian is a force to be reckoned with as a person, so he just brings so much that there isn’t much effort I have to put forward.”
Just like Roman, the actor finds himself genuinely intimidated by Cox and is eager to impress him. He concedes that Cox is “a wonderful guy,” but he has his “Logan-y” moments. Sometimes Culkin is forced to ask, “‘Is that Logan or is Brian just hungry? Can someone get him a sandwich? He’s about to snap at you.'”
Cox is certainly a formidable scene partner, but Culkin may be the closest “Succession” cast member to matching the sharpness and conviction of Cox’s wit. The characters share a love of the F-bomb, given that Logan’s catchphrase is “F*** off” and Culkin finds himself saying the word in “every sentence, more or less.” Culkin finds Cox’s strong presence “rubs off” on him, causing him to act differently both on and off set. Just like his character, he seems to act more like Logan when the man is not around.
Perhaps the relationship between the two actors bleeds into the one between their characters. Culkin’s willingness to give in to Cox’s dominance is perfectly suited to his character, who is often the first of his siblings to bend to his father’s will. While their real-life dynamic is of course friendly, it’s easy for Culkin to put himself in Roman’s shoes. Complex dynamics like this one imbue “Succession” with its lovable naturalistic edge.
Source : https://www.slashfilm.com/889876/according-to-kieran-culkin-he-has-a-lot-less-work-to-do-in-succession-scenes-with-brian-cox/