LOS ANGELES (AP) — Jessica Chastain doesn't even feel comfortable saying her character's name.
The Oscar-nominated actress quickly attempts to course correct when she realizes she's speaking liberally about Murph, the astrophysicist she's portraying in "Interstellar," one of the most closely guarded and anticipated films of 2014.
"I'm just so terrified," she says sheepishly during a recent interview to promote the film, which opens next week. "It's obvious, right?"
While only a 7-year age difference separates Chastain from Matthew McConaughey in the real world, the theory of relativity finds them as a disconnected father and daughter in Christopher Nolan's sweeping space-time saga about a last-ditch effort to find humans a new home in another galaxy.
The film takes place in the near future after Earth has been ravaged by a blight that's left many food sources extinct. McConaughey's widowed pilot-turned-farmer Cooper is tasked with leaving behind his young son and daughter (played by Timothee Chalamet and Mackenzie Foy) for a space mission through a wormhole to planets that might be fit for humanity.
"If you're a parent or even if you just have a parent, everyone has these moments, from lesser to extreme levels," said McConaughey. "It happens all the time, whether you're dropping your kids off at school or going on a vacation. This is the most extreme nature of that. This is a father going off for a long time. There's no guaranteed return ticket."
It would be spoilery — and cerebrally taxing — to explain just how and why the 44-year-old "Dallas Buyers Club" actor and the 37-year-old "Zero Dark Thirty" actress simultaneously end up as father and daughter in the film. Yet it can be said the complexity of it all had Chastain feeling isolated from the cast and crew during production.
"I didn't realize it at the time," said Chastain. "As an actor, you create an environment where you do your work. I was isolating myself, but it wasn't until we were traveling and having discussions recently with Chris (Nolan) and everyone else that I recognized, 'Wait. This never happened on set.' I think now that it was probably intentional on Chris' part."
Murph was initially envisioned as a boy in the original script by Jonathan Nolan, the filmmaker's brother, when director Steven Spielberg was first orbiting the project. It wasn't until Christopher Nolan took control of "Interstellar" as cowriter and director that Murph morphed into a girl and the top-secret film was nicknamed "Flora's Letter," a reference to the director's daughter.
"By changing the sex, it made it more complex," said Chastain. "We've seen many Hollywood stories about a son becoming a man with his father's help. That's almost every journey in cinema. It's rare we see the dynamic between a father and a daughter. If you're supposed to be protected, and you're left behind, what kind of relationship does that create?"
Despite stretches of space separating their characters, both actors needed to wrap their heads around difficult subjects like physics and cosmology: Cooper is a tinkering MacGyver-like engineer, and Murph grows up to become the NASA protege of the same professor (portrayed by Michael Caine) who convinced her father to get lost in space. (Casey Affleck plays the adult rendition of Cooper's son.)
"When I watch the film now, I still don't understand everything in it, but the main part of this film isn't about science," said Chastain. "It's about love. You have to feel it. If you go into the movie — even though the scope is large with space travel — at its core, it's a story about a father and a daughter. If you let it wash over you, that is enough."
The estimated $165 million production opens in U.S. IMAX theaters on Wednesday and goes wide next Friday. Industry tracking research indicates the film could earn more than $55 million domestically on its first weekend out.
Follow AP Entertainment Writer Derrik J. Lang on Twitter at http://www.twitter.com/derrikjlang .