Julia Roberts calls "Osage" her best acting experience

An exuberant Julia Roberts calls working on her latest film, "August: Osage County," "the best acting experience of my life."

The 46-year-old actress was bubbling with enthusiasm the day after the highly anticipated film adaptation of Tracy Letts' Pulitzer Prize-winning play premiered at the Toronto International Film Festival. In the film, Roberts plays the eldest daughter of a brutally honest matriarch, played by Meryl Streep.

"I've never worked so hard in my life -- and I've given birth to three children," Roberts told reporters Tuesday.

The large cast (which also includes Benedict Cumberbatch, Abigail Breslin, Ewan McGregor and Dermot Mulroney) lived near the on-location set in Oklahoma, devoting themselves to capturing Letts' dialogue. The film, to be released Dec. 25, drew mixed reviews at the festival but praise for its performances.