In theater, as Rebecca Hall fully understands, you stick to the script. In film, not so much.

That goes for the process as well as the production, as the actress learned after being spotted by Woody Allen’s casting director, Juliet Taylor, in a New York staging of “As You Like It.”

From the moment the “Vicky Cristina Barcelona” director called her in, the unexpected became a way of life.

“We met for about three minutes,” Hall says. “We had a really sort of superficial discussion about green jumpers, the weather and St. Patrick’s Day or something.”

Hall, figuring she didn’t make any sort of impression with the 72-year-old and three-time Oscar-winning director-screenwriter-actor, says she didn’t hear from Allen for a year. Then, the phone rang.

“He said, ‘We met before,’ and I went, ‘Yeah, I remember,’ ” recalls Hall, who speaks with a thick British accent. “He asked, ‘Can you do an American accent?’ I said, ‘Yes.’ He said, ‘OK, bye.’

“I didn’t know that there was a job. … Two weeks later, I got a script in the post.”

The casting experience was only the beginning of Hall’s lessons in working without a net. When filmmaking began, Allen tasked Hall to improvise and make the Vicky character her own. “Initially terrifying, but I loved it,” Hall says of the experience. “I found it enormously liberating.”

Hall also makes an impression in the upcoming political pic “Frost/Nixon,” for which things went more according to plan. But even then, Hall had to reconcile what she called the “fictionalized reality” of the screenplay.

She says that before writing the script, Peter Morgan — the Oscar-nominated screenwriter of “The Queen” — didn’t meet Hall’s character, Caroline Cushing. But Hall did.

“There were things she would tell me about the whole events as she remembered them that did not add up with the script,” Hall says. “You have to separate them. It’s not the story of Caroline Cushing; it’s the play devised by Peter Morgan about these interviews that took place.”

Hall, the daughter of opera’s Maria Ewing and English theater director Peter Hall, plans to stay onstage as well — her next production is “The Bridge Project.”

Though she has only been working in features since 2006’s “Starter for 10” and “The Prestige,” her film sked is now overloaded, with pics including “Dorian Gray,” an untitled Nicole Holofcener project and “Red Riding.”

“I grew up in the theater,” she says. “I always sort of imagined I would be a theater actor, but I always had this borderline obsessive, geeky thing about films and have watched everything, really.

“You just hear so many stories that it’s so hard to break in, (and how) it’ll never happen for you. When it started happening, it was surprising.”

By now, Hall realizes, surprises come with the territory.

An actor should always:
“Brush his teeth. No that’s awful. But, yeah, actually, they should.”
Lucky break: “Just being in the right place at the right time.”
Favorite film character: “There are too many. It’s either going to be the character Ruth Gordon played in ‘Rosemary’s Baby’ (Minnie Castevet) or Margo Channing (Bette Davis) in ‘All About Eve.’ “