In 2000, Mortimer was cast as Katherine in Kenneth Branagh's musical adaptation of "Love's Labour's Lost", where she met actor and future husband Alessandro Nivola. She took on her biggest role in an American film to date, playing opposite Bruce Willis in "Disney's The Kid". A year later, she played aspiring actress Elizabeth in "Lovely & Amazing", a comedy about the relationship between a mother and her three daughters. Mortimer said of the role, "It was a wonderful experience as an actor to have that opportunity [...] You hear this terrifying phrase, 'being in the moment.' I have no doubt that I was in that moment. was exposed and ridiculous and brave." Mortimer won an Independent Spirit Award for Best Supporting Actress for her role. In 2002, she had a major part as the assassin in "The 51st State" (also known as "Formula 51"), starring opposite Samuel L. Jackson and Robert Carlyle. Upon release, the film was a critical and commercial failure. 2003, Mortimer appeared in Stephen Fry's British drama, "Bright Young Things," based on the 1930 novel "Vile Bodies" by Evelyn Waugh. It is about young and carefree London aristocrats and bohemians, set in the interwar period. Playing fiancée Nina Blount, Mortimer was described as "a character who gives Peter O'Toole a marvelous few minutes of playing dottiness to the hilt", by "Washington Post"'s Stephen Hunter. Her last release of 2003 was "Young Adam", in which she plays the girlfriend of a cruel barge worker (Ewan McGregor). Based on the novel of the same title, Mortimer's role in "Young Adam" garnered her acclaim; "Evening Standard"'s critic wrote, "In a performance of naked courage, Emily Mortimer shows that she is prepared to go to extreme lengths of masochism in her depiction of a woman undone by love". "Sight & Sound" magazine thought the cast performances were "edgy", and Mortimer made the most out of an underwritten role. She also had a supporting part in the romantic drama "The Sleeping Dictionary" (2003).