Private and unassuming in her personal life, Annette Bening relished the idea of going diva in “Being Julia.”

“The part was so rich — a feast of words and character and ideas,” says Bening, who already this awards season has been named best actress by the National Board of Review and has a Golden Globe nom.

“Most actors behave how a character would behave — their mannerisms, the way they stand or sit, or whatever,” says the pic’s screenwriter, Ronald Harwood. “But very often they don’t reach the depth of the character that Annette has done. There is a difference. She digs into the lines to find the subtext.”

And with Julia Lambert, there’s plenty of subtext.

“It’s an impossible relationship she finds herself in the midst of with this young guy,” says the 46-year-old Bening. “But it’s a necessary crisis to get her onto the next part of her life. This story really takes you to a place where you understand, ‘Oh, I see, she’s actually shedding a skin here. She’s actually coming out better for the whole thing.’ ”

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To prepare for the part, Bening immersed herself in pre-World War II British theater. She listened to lots of tapes to get the accent down, and studied paintings and pictures from the time — but only particular ones.

“There’s lots of movie stuff from that period and photographs of movie stars and movie sets,” the two-time Oscar nominee says. “But this is a woman who’s not in that world. Her world takes a decidedly different context.”

There also was the source material, W. Somerset Maugham’s novel “Theatre.” In addition, Bening pulled from her own stage experience, which includes studies at San Francisco’s American Conservatory Theater.

Bening’s background, plus nearly a year of prep work, resulted in a nuanced performance that was on-target, Harwood says.

“We had a reading early in the process and she was absolutely dead center in the part from the moment I first heard her read it. Annette absolutely understood the kind of actress Julia Lambert is and the kind of woman she is — and that’s the secret of the role.”