For approximately half of period drama “Vera Drake,” shuddering despair overwhelms the title character played by Imelda Staunton, carrying her away from former happiness like an undertow.

It is such an unremitting sadness that one might fear for the well being of the actress compelled to portray it. And that’s even before realizing that under the direction of Mike Leigh, who works without a script and obliges his performers to improvise and create their own characters, rehearsal and filming of “Vera Drake” took nine months.

Staunton plays a woman who is arrested in 1950 for performing an abortion in violation of the law. But despite the tragic intensity required by her performance, the biggest role of her career, the actress went home cheerful every night.

“Mike’s very good and very structured and very healthy,” Staunton says. “His way of working is you do it in your work, and you don’t take it home. You come out of your character and the character is left there. You walk away and I found that easy.

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“I think the only thing is, I think we spent Monday through Saturday doing all the scenes being interrogated, and I must say my nose was very red.”

Even with the rosiest nose since Rudolph, Staunton found her experience with Leigh exhilarating, saying she was “completely empowered” by the preparation time and individual attention Leigh gives his actors.

“The preparation is so fantastic,” she says, “you’re not going home thinking, ‘That wasn’t right. What was I doing?’ A lot of energy is spent thinking it’s not right. But I never felt frustrated.”

It also helped that Staunton, a veteran of such films as “Sense and Sensibility” and “Shakespeare in Love,” was prepared for an involved commitment. She savored it.

“It was just the best job of my life,” Staunton recalls. “Every day was an adventure, and I really lapped it up. Sometimes it felt like you were leaving the plane with no parachute — and he was there next to you.”

Staunton now finds herself floating toward an Academy Award nom, something she finds both thrilling and daunting.

“It just gives me indigestion, quite frankly,” Staunton says. “It’s fun. It’s wonderful.”