Geoffrey Rush in 'The Diary of a Madman'

Geoffrey Rush is giving one of the most astonishing performances of ordeal theater we're ever likely to see.

Even for this daring actor, Rush - who has an Oscar nomination for eccentrically curing a stammering king in "The King's Speech" and a Tony for portraying unhinged royalty in "Exit the King" - goes deliriously over the edge this time.

He is riveting and exhausting. Alas, the play, "The Diary of a Madman," is just tiring. Adapted from a Nikolai Gogol story from 1853 Russia, the piece - virtually a two-hour monologue (with one brief break) - charts the unraveling of a minor civil servant from antic grandiosity to equally animated insanity.

Whether one finds this thrilling big-gesture theater or overbearing preciosity will be a personal taste. Those of us with a low whimsy threshold may find more to admire than to adore.

Rush plays a preening seedy fop of a creature with an embalmed pallor and a poof of clown-orange hair on his bald forehead. He lives in the grimy attic of a boardinghouse painted in garish crayon colors. Convinced that he deserves to be much more than a clerk, he keeps a daily record of life's indignities, the inferiority of his superiors and the shame of wearing a coat with unfashionable lapels.

Rush, for all his success in movies and TV, is in his element as a fabulist in the theatrical absurd. It is a style honed at Australia's Belvoir, where Rush has long collaborated with artistic director Neil Armfield.

Their "Exit the King" was profoundly moving on Broadway two years ago. But that was a multicharacter play meant for the stage. "Madman," co-adapted by Rush, Armfield and David Holman, is a showpiece of an internalized piece of fiction.

A cheerful actress named Yael Stone scampers around as a Finnish housekeeper with language issues, as the clerk's fantasy love and, finally, as a fellow inmate in the asylum. Also, two excellent musicians use strings and wind instruments to interact with him, and beautiful shadows loom.

But this is a show for Rush - all shape-shifting elbows and knees and mellifluous articulation - who impersonates a cricket, talks to dogs, dances around and goofs with the audience. Many will appreciate the fullness of the performance. Others may find it a long sit.

WHAT "The Diary of a Madman"

WHERE BAM Harvey Theater, 651 Fulton St., Brooklyn

INFO $35-$95; 718-636-4100; bam.org, through March 12

BOTTOM LINE Geoffrey Rush showpiece