Who hasn’t considered stepping away from the lives we find ourselves in? The daydream washes over us while we’re at work or waiting in line at a coffee shop. Maybe it comes from seeing someone — an intriguing figure wearing surgical scrubs, perhaps? — who makes us wonder what it would be like to stride confidently into an emergency room and assess the scene, then take command of a team of professionals in order to save a life. Or maybe the fantasy is less glamorous, rooted in the solitude that comes from conducting esoteric research in isolation, hoping the vital data amassed has the potential to impact thousands later.

That’s what makes acting so fascinating. For the duration of a shoot, a film actor steps into a role, an entirely different persona. Sometimes that effort is shaped specifically by character traits, psychological triggers and emotional responses. In other situations, characters are molded based on tasks, allowing an actor to immerse himself or herself in a divergent field of study. The opportunity exists to approximate what it means to be a politician, physician, dancer or an athlete, and to do so with authority.

In Complete Unknown, the new release from Maria Full of Grace writer-director Joshua Marston (who co-wrote the script with Julian Sheppard), we can see such a journey occur for both the actors and the characters they play. The film progresses through a series of shifting tableaus featuring Rachel Weisz in a parade of roles. We see fragments of distinct lives and it would be easy to assume that she’s going through them as if they might be garments off the rack, items she can slip in and out of, as she’s trying to find that one perfect fit. But, slowly, even in these brief sketchy flashes, we sense how she more fully inhabits each role, investing them with meaning and specificity.

She is not merely a tourist or a supporting player in someone else’s story. In each role, she claims center stage, as we all do in our own lives. In her current incarnation in the film, she is Alice, a researcher who insinuates herself into the life of a man who happens to be the friend and co-worker of Tom (Michael Shannon), a former lover who knew Alice back when she was Jenny, before she started escaping into other lives. 

When Alice and Tom finally meet, at his birthday party, she catches him off guard, but also at a time when stepping away might have a certain appealing allure for him. He feels trapped by the decision of his wife Ramina (Azita Ghanizada) to leave the East Coast to attend graduate school out west. 

He supported her desire for change without fully considering the impact it would have on his own work, and now he’s questioning whether to stay in his comfortable situation, leave with his wife or, after taking off on a night-long excursion with Alice/Jenny, head off into the unknown with her.

Tom and Alice share a couple of quiet conversations, catching up with each other in a way that, on a certain level, is fraught with tension and confusion. But when Alice breaks free of the party, Tom follows her. The pair embarks on an interlude during which he gets the chance to sample what it is that Alice does, and how easy it is to take on another identity. 

They help an older woman (Kathy Bates) in the street and walk her home, extending the chance exchange when they interact with the woman’s husband (Danny Glover). Tom slowly adjusts to the new role he’s taken on, and Shannon’s acting illustrates for us just how gradual and discomforting the transition can be. But we also see him settle in.

Of course, the moment of truth arrives when both Tom and Alice must decide what to do. Can she finally embrace one identity and the routine happiness it might provide her? Marston’s narrative wants us to imagine she is at the end of her journey while Tom may be ready to take flight. So much of life and relationships is about the combination of finding the right person at the right time. 

Complete Unknown tweaks that idea, shifting the focus from the search for another person who might complete us to discovering the best and most complete version of ourselves. It is an intimate high-wire act, completely open to interpretation, but with Weisz and Shannon as our guides, Complete Unknown doesn’t leave us stranded alone. (Opens Friday at Esquire Theatre) (R) Grade: B+