1997 marked Ifans' breakthrough starring alongside his real-life younger brother Llyr Evans (Rhys adopted the Welsh spelling of their surname) as sociopathic brothers in "Twin Town". Presumably setting out to do for Swansea what "Trainspotting" did for Edinburgh (and even executive produced by Danny Boyle and Andrew Mcdonald), the less moralizing "Twin Town" proved a mixed success, with many finding the subversive elements of the film troubling, while others enjoyed its boundless energy and irreverence. Ifans' grimace laden portrayal of the gleefully violent Jeremy was at once enrapturing and disturbing. The following year, his supporting role in the Irish drama "Dancing at Lughnasa" would showcase a different side to the actor. As the dreaming free spirit Chris, father of an illegitimate son with Catherine McCormack, Ifans deftly played a much more likable and inspiring role, the handsome and endearing dreamer whose pure-heartedness sparks the sisters' sense of independence and abandon.

While his previous performances were strong and compelling, they did not capture the public notice that would meet his turn as the unwashed oddball Spike, his scene-stealing portrayal adding just the right measure of inane comic relief to the well-received romance "Notting Hill". Ifans played the roommate of an unsuccessful bookshop owner (Hugh Grant) linked to a world famous actress (Julia Roberts). Spike's inadvertent efforts to keep the lovers parted and his original way of dealing with the ever-present press were highlights of an already enjoyable film. Unwashed, unshaven and just plain unbelievable, Ifans was virtually unrecognizable in the role (for which he prepared by going unkempt in real life). He cleaned himself up for a starring turn in "Rancid Aluminum" (also 1999), a darkly comic crime caper co-starring Joseph Fiennes, Tara Fitzgerald and Sadie Frost. In this adaptation of the novel by Welsh author James Hawes filmed in Wales and Eastern Europe, Ifans played a young businessman who mistakenly takes up with the Russian Mafia in a bid to end his financial woes, with disastrous results. The busy performer also completed turns in "You're Dead" (lensed 1998), a thriller starring John Hurt, "Janice Beard: 45 wpm" (1999), a romantic comedy-drama in which he is a ne'er-do-well co-worker of the title character (Eileen Walsh), and "Heart" (both 1999), a drama centering around an organ transplant starring Christopher Eccleston, and the football comedy "The Replacements" opposite Keanu Reeves and Gene Hackman. His next role in a maninstream Hollywood film was as Adrian, one of Adam Sandler's envious fellow sons of Satan, in the comedy "Little Nicky" (2000).

The actor snared one of his most chalenging, if little-seen, roles when he starred as Puff, a man raised in the wild by apes, in the absurdly offbeat "Human Nature" (2001), a screenplay written by Charlie Kaufman--a specialist in the unusal--and directed by Michel Gondry. Also in 2001, Ifans was cast in the little seen UK drug comedy "Formula 51", starring Samuel L. Jackson, and landed the role of Beaufield Nutbeem in director Lasse Halstrom's disappointing adaptation of the bestselling novel "The Shipping News"; the following year he had a supporitng turn in the UK comedy "Once Upon a Time in the Midlands" (2002). Ifans had a brief hiatus from the big screen before returning in the offbeat Australian charmer "Danny Deckchair" (2004) as the title character, who through a mishaps floats away from his unsatisfying life in a lawn chair hoisted by helium balloons and crash-lands miles away to discover a new life, and a new love, awaiting him--even as the media descends on his story and he tries to keep his true identity a secret. That same year saw Ifans take on the role of the love-starved Dobbin for director Mira Nair's big-screen adaptation of Thackeray's "Vanity Fair," starring Reese Witherspoon.