Roger Ebert often said that what matters is not what a movie is about, but how it’s about it. Viewed from that angle, Joshua Marston’s Complete Unknown largely fails to find a compelling way to be about its subject. Alice (Rachel Weisz) is a habitual impostor in the Catch Me If You Can vein. She’s been, among other things, a nurse and a magician’s assistant in China, and is currently a frog scientist in New York. Attending a party with a fellow frog scientist, she encounters Tom (Michael Shannon), who recognizes Alice as a woman he knew 15 years earlier named Jenny; Tom becomes obsessed with getting the truth out of Alice, to the possible detriment of his marriage to the much more honest Ramina (Azita Ghanizada). The majority of Complete Unknown is a two-hander between Weisz and Shannon — whose chemistry here feels wasted — and their conversations about the impact of her disappearance on those she left behind are more interesting than any of her identity-chicanery. The picture never fully reckons with the ethical issues raised by her fundamentally dishonest life, thus by extension excusing them — and further demerits are added for the title’s crib from Bob Dylan’s “Like a Rolling Stone,” an association Complete Unknown doesn’t come close to earning.