These days, Justin Theroux is probably best known as Mr. Jennifer Aniston.

That’s more a reflection on our unceasing obsession with celebrity than it is Theroux’s own talents as an actor and occasional screenwriter, which are considerable and have spanned everything from “Mulholland Drive” and “The Leftovers” to his work co-writing the immortal “Tropic Thunder.”

Beginning Friday, the 45-year-old Theroux would be hard to miss: He’s playing Tom, ex-husband to Rachel (Emily Blunt), the Westchester-to-Manhattan commuter at the center of “The Girl on the Train,” the bestselling Paula Hawkins novel that’s been turned into a big screen feature from director Tate Taylor (“The Help”).

The story, rich with twists and turns, is immersed in the confused psychological states of Rachel and its two other protagonists, Megan (Haley Bennett) and Anna (Rebecca Ferguson). That’s tough terrain for any actor, so Theroux needed to key on an essential throughline to make sense of Tom.

“No one in this story is operating from an elevated emotional place, or very emotionally realized place,” Theroux says. “There’s a lot of damage to go around and Tom is no exception.”

Theroux says he admired the book and the screenplay and speculates that his screenwriting background (his credits in that area also include “Iron Man 2”) helps him read things differently than your everyday actor.

“I think if there’s any benefit to writing screenplays ... is the ability to sort of go to 40,000 feet and see the thing as a whole, and realize this is not a film that’s about me or my character,” Theroux says.

“[It’s] the real desire to service just the screenplay, actually director first and then screenplay, to service whatever their vision is, whatever they want to do,” he continues. “Some actors can not have that perspective and not understand that it’s a house of cards and they can try to falsely draw the material to them in a way that’s not good for the overall piece.”

Theroux got his start in hit late ’90s films like “Romy and Michele’s High School Reunion” and “American Psycho,” before things began really taking off in the seminal “Mulholland Drive,” David Lynch’s 2001 masterpiece that critics polled by the BBC recently called the best film of the 21st century to date.

Lynch, Theroux says, taught him a key lesson about preserving the sanctity and mystery of the movies, with some key advice before the film’s press junket.

“If you are asked about this film or what it means, don’t answer this question. Because you’re going to put things into their heads, even with best intentions, that deprive them from the ride I’ve created. ... Inevitably by trying to explain it, you damage it.”