Kenneth Branagh (search) knew that Franklin Delano Roosevelt had polio, but like most people, he was unaware of how he dealt with the disease — first trying to conquer the disability, then learning to live with it.

After reading Margaret Nagle's carefully researched script for "Warm Springs" (search) (premiering 8 p.m. EDT Saturday on HBO) Branagh felt it was "about a life-changing moment, and a character-changing moment ... What is very recognizable in this story is the human quality behind a man who was iconic."

Although paralyzed from the waist down, FDR largely kept his disability from public view during his presidency (1933-45).

The film — which co-stars Cynthia Nixon as Eleanor Roosevelt and Jane Alexander as FDR's mother, Sara — focuses on Roosevelt's life between when he first contracted polio in 1921 at age 39 and the late '20s.

The show's title refers to the Georgia health spa where Roosevelt sought treatment from its mineral-rich waters and met other polio sufferers born into circumstances very different from his own life of privilege. He used his trust fund to buy the run-down facility and refurbish it, establishing a viable institution that still functions today. (Part of the film even was shot on the premises.)

During the seven-year odyssey depicted in "Warm Springs," Roosevelt tries everything in the hope of walking again, says Branagh. "He's also going through all the cycles that seem to be the normal consequences — tremendous depression, tremendous anger, tremendous apathy, as people seem to have before finding their motivation."

Branagh studied thousands of photos and film clips — even though they were carefully crafted to project the public image of confidence and vigor, not a man with crippled limbs and chronic pain.

There are only a few known photos of FDR in a wheelchair, he says, and only "about three or four seconds" of film showing "his waddling walk" — where he holds a cane in one hand and his son's arm with the other.

Branagh watched that footage repeatedly. "It's a kind of a trick or stunt ... just for those special moments when they wanted to say to the American public, `Yes, he's had polio, but look, he's walking.' It was a very painful and effortful struggle, and he couldn't do it for long because he sweated enormously, so that would give the game away."

Branagh also had to capture FDR's voice. Roosevelt's public delivery is famously known from his Pearl Harbor speech and "fireside chats," but far less is known about his private speaking style. So the actor says his interpretation was "obviously a bit of a guesstimate," applying the notion that Americans of Roosevelt's class, culture and era did sound somewhat English.

HBO Films President Colin Callender thinks Branagh succeeded in capturing the full range of FDR.

"Ken can play the paradox in a character, so he can play the vanity and the vulnerability at the same time and that's what this role is," says Callender, "the self-assurance and the self-doubt."

Branagh, 44, was born in Belfast, Northern Ireland, but moved to Reading, England, when he was 9 — close to where he lives now. Once married to Oscar winner Emma Thompson, he's now wed to artist Lindsay Brunnock, whom he met when she worked as an art director for "Shackleton," the A&E movie in which he played the famous explorer.

As a youth, Branagh's first love was sports. "I played soccer and would have loved to do that, but wasn't good enough by a million miles," he recalls.

Corralled into a school production of the musical "Oh! What a Lovely War," he played about a dozen parts and "it just seemed like such fun."

He's received four Oscar nominations, including for starring in and directing 1989's "Henry V." But perhaps a more significant recognition for Branagh is that he's often asked by kids to sign autographs as the vain Professor Gilderoy Lockhart in " Harry Potter and the Chamber of Secrets."

"They're sweet," he says. "They have no idea who you are. They go, `You're the one with Cornish pixies.'"