By day, the man haunts the corridors of New York’s Port Authority bus terminal, methodically retracing the steps of the moment, some months earlier, when his 6-year-old daughter disappeared, never to be seen or heard from again. By night, he numbs himself with drink, drugs and cheap bathroom-stall fucks — before morning comes and brings with it a renewal of his relentless search. The man is called William Keane, and he is the title character of the stunning new film by writer-director Lodge Kerrigan, whose 1994 debut feature, Clean, Shaven, also told the story of a father’s search for a missing daughter. Only, in that case, it wasn’t a kidnapping, but rather the father’s treatment for a schizophrenic condition, that was cause for the separation.But Keane, too, is a portrait of psychological instability, and as the film progresses, it asks us to consider whether its protagonist’s extreme emotional trauma is a byproduct of his mental illness or vice versa — indeed, whether William Keane actually has a daughter at all. Those contradictions are embodied by the 34-year-old British actor, Damian Lewis, who appears onscreen for every one of the film’s 90-odd minutes and who commits himself to the role with terrifying intensity. His Keane is by turns clear-headed and delusional, paternal and childlike — whether having a conversation with himself or going into a strange trance to the strains of “I Can’t Help Myself” emanating from a barroom jukebox. For Lewis, the film is a breakthrough coming after an acclaimed supporting turn in the Band of Brothers miniseries and an impressive dual role in Lawrence Kasdan’s Dreamcatcher (where his Jonesy/Mr. Grey is one of the occasional flashes of brilliance lighting up that otherwise daft Stephen King adaptation). In Keane, he is the engine that drives an utterly relentless film. And we cannot escape him, for Kerrigan’s handheld camera is forever hovering no more than a few inches from Lewis’ head, forcing us to see the streets of New York through the eyes of a man we would ordinarily do our best to avoid. It is one of the year’s great pieces of movie acting, and the kind of performance other actors come to speak about in hushed, reverential tones.“I watched a lot of videotape of people suffering from mental illness,” says Lewis, who also has a small part in the long-delayed Robert Redford-Jennifer Lopez vehicle, An Unfinished Life, which opens today. “Then, of course, we were shooting in the Port Authority, on live locations, and that place is just packed to the rafters with homeless and delusional people. At the risk of sounding like I was abusing their illness, it was incredibly useful. Once you imagine an anxiety, that people are staring at you or talking about you, and you find yourself in an environment where thousands of people are walking past you and brushing up against you all day long, it’s very easy to become frightened and panicky. I think I learned from doing this job that it’s very easy to flip out.”Yet while Keane is a character rooted in uncertainties, Lewis notes that, as a performer, he found it essential to have very concrete ideas about the role. “If we accept the premise that maybe this young girl does not exist, nevertheless the actor playing the character and in fact the character himself must absolutely believe that she exists,” he says. “The two, in terms of process for the actor, are the same — whether she’s actually there or he just believes she’s there. There is no mileage to be gotten from playing the ambiguity of the situation.”Lewis also has only the highest words of praise for Kerrigan — “You don’t often become best friends with your directors, but Lodge and I have,” he says — and for his claustrophobic camera work. “Having someone that close to me and really never having the camera face-on to me — it was always in profile, or directly over my shoulder — it became like a dance. I felt so concentrated in my role that I rarely found it intrusive. Acting, at its best, is sort of meditative, actually. In discovering a character, you lose yourself.” Keane opens in Los Angeles theaters on Friday, September 16. Check back next week for Ella Taylor’s review.