NEW YORK (Reuters) - Playwright Tracy Letts created the plot and characters in August: Osage County, the Pulitzer Prize and Tony award-winning play about a dysfunctional Oklahoma family, and knew that adapting it for the screen would not be easy.

It was hard work condensing a three-hour play that premiered on Broadway in 2007 and enthralled audiences into a two-hour film, but Letts, 48, was grateful to be able to do it.

"There is a reason that impels you to write a play in the first place and I was afraid that in the hands of another writer some of that reason might be lost," he told Reuters in an interview.

The film, which opens in U.S. theaters on Christmas Day with a star-studded cast headed by Oscar winners Meryl Streep and Julia Roberts, poses questions about family, its meaning and whether to battle with relatives or walk away when things get tough.

"I thought they were valuable questions to ask," he said.

Letts, also an actor who won a Tony Award this year for his portrayal of George in a revival of Edward Albee's Who's Afraid of Virginia Woolf?, grew up in Oklahoma.

He drew on events and people in his own life when writing August: Osage County, which is filled with twists and surprises and deals with addiction, betrayal, resentment and adultery with doses of humor as well as piercing sadness.

Letts said his own experience as an actor informed the characters he created, including the chain-smoking, pill-popping, acid-tongued family matriarch Violet Weston, which earned Streep, 2012's Academy Award best actress winner for The Iron Lady, her 28th Golden Globe nomination.

"I think I know the thing that actors want, not just want, but need in a role," said Letts.

Roberts, a 2001 best actress Oscar winner for Erin Brockovich, also clinched a Globe nomination, her eighth, for her role as Barbara, the eldest and most combative of Violet's three daughters, in a role she described as the most difficult of her career.

"It was the hardest by far. Everything was hard," she said.

FAMILY SECRETS AND LIES

The focus of August: Osage County is a gathering of the Weston clan in the oppressive heat of summer following the suicide of Violet's hard-drinking, poet husband, played by Sam Shepard (The Right Stuff).

Violet, suffering from cancer and addicted to pills, is unrelenting in her attacks on family members, particularly Barbara, spewing venom every chance she gets.

Julianne Nicholson (Kinsey) is the shy, lonely Ivy, the single daughter who remained in Oklahoma and is having a secret romance with her awkward, insecure cousin Little Charles, played by Benedict Cumberbatch (The Fifth Estate).

Ewan McGregor (Trainspotting) is Barbara's estranged husband, who is having an affair, and Abigail Breslin (Little Miss Sunshine) is her pot-smoking, teenage daughter.

Rounding out the stellar cast are Juliette Lewis (Natural Born Killers) as the flighty daughter and Dermot Mulroney (Jobs), as her sleazy lover, along with Margo Martindale (Million Dollar Baby) Violet's sassy sister, and Oscar winner Chris Cooper (Adaptation), who plays her cuckold husband.

For Roberts, getting to the core of Barbara and the reasons for each conflict was like an excavation project.

"There is a little glimpse into one person's cruelty that gave a spark to the next person's cruelty, and when someone is that mean to you how do you not take part of that as your way of dealing with people?" she said. "It's complicated. It's so ugly."

Letts is pleased that the play and the film both exist but said seeing each is a very different experience.

"An audience has to work a little harder in the theater," he said. "In the movie it all comes to them."

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Julia Roberts, Meryl Streep