As is outlined in one of these TCM articles, Gregory Peck, whose contracted stipulated sole star billing, called the powers that be at the studio and insisted that Audrey Hepburn be given star billing with him. Ever the gentleman, Mr. Peck would later claim that it was merely enlightened self-interest - that he would look ridiculous being labeled as the only "star," when Ms. Hepburn so clearly dominated the film. But it is also a mark of his total professionalism and dedication to his craft that he so quickly recognized the birth of a new star.
Hepburn was not the first choice for the part - it was considered as a role for Jean Simmons, Elizabeth Taylor (the first choice of the Frank Capra, who at one point was going to direct), and Suzanne Cloutier (who was screen tested by Wyler). Cary Grant (who declined the part) was the first choice for Joe Bradley (thankfully - he and Ms. Hepburn would work together FINALLY in Charade. They were a match made in heaven!). For more detail on the background of the film, see this extensive article in the AFI Catalog.
My Love Came Back (1940), The Sun Also Rises (1957), and Oklahoma (1955). His career might have been even more substantial had he and his wife, the actress Margo (they were married from 1945 until her death in 1985), not been caught up the Hollywood Blacklist. Eventually, Albert segued into television - most famously in the series Green Acres, but also in Switch and a daytime variety show called The Eddie Albert Show. He lived til age 99, dying in 2005 of Alzheimer's disease.
Eddie Albert wasn't the only person on this film touched by the Blacklist. In 1992, AMPAS finally awarded to Dalton Trumbo his Oscar for Best Motion Picture Story, which had previously been credited to Ian McClellan Hunter (who himself was later blacklisted) as sole author. Having just seen Roman Holiday, it was interesting to also see Trumbo, which goes into some detail about the help Hunter provided in getting Trumbo's screenplay to film.
National Registry of Historic Films as well as being number 4 on the AFI's 100 Years, 100 Passions list. It's a lovely film, with a visualization of true emotions and adult responsibilities. I'm going to close with perhaps the most famous scene in the film, the Mouth of Truth. Allegedly, Mr. Hepburn didn't know that Gregory Peck was going to pretend his hand had been bitten off, and her quite convincing scream was real. Regardless, it's a joy to watch, and expertly done. Enjoy!