Though PEOPLE film critic Leah Rozen found Liam Neeson's a "more sympathetic" screen character than his "K-19: The Widowmaker" costar Harrison Ford's stuffy commander, the Irish star admitted that it was difficult to keep a lid on his sense of humor while making the serious Soviet submarine drama. Part of his clowning around, he told reporters (and reported by the Associated Press), may have been spurred on by the claustrophobic nature and setting of the Cold War tale. "You have to make a lot of jokes, you know. You just have to, you have to keep a lighter side," said Neeson, 50. "Because you know you're going to go into sort of a deeper side." (There may have been a pun in that last remark.) As for how his director, Kathryn Bigelow, reacted to Neeson's lighthearted behavior between takes, Neeson revealed: "I think it may have frustrated Kathryn in the early days, the fact that we were always cracking jokes and thinking perhaps we weren't taking this seriously. But you do it as a little antidote, knowing you are going to get intense." Talk about being in over your head.