Few figures from Hollywood's Golden Age loom as large as Orson Welles.

The mercurial wunderkind first made a name for himself with his voice, in his Mercury Theater's Shakespeare productions and his iconic radio adaptation of The War of the Worlds. Then he took Hollywood by storm as a creative force, acting, directing, and producing his own work, including his most famous film, Citizen Kane, which is central to David Fincher's new movie Mank.

As Welles aged, he became larger than life, both in his increasing girth and his tempestuous approach to filmmaking, a realm in which he was often at odds with studio brass and even himself (see: the long-unfinished The Other Side of the Wind).

How then do you find the right person to portray such a towering figure? It's been done before several times, by the likes of Vincent D'Onofrio, Angus MacFadyen, and even Jack Black.

But in Mank (now streaming on Netflix), Welles is a foil to Herman J. Mankiewicz, the quixotic alcoholic screenwriter played by Gary Oldman. Welles is a rising director on the cusp of triumph, with an ego to match. He's neither the young man portrayed in films like Me and Orson Welles nor the Falstaffian creature of later years as seen in Ed Wood. He is Orson Welles at the height of his powers, with one of the most recognizable faces and voices ever put to screen. And those are big shoes to fill.

Mank director David Fincher tells EW he set out to find his Welles with a specific note in mind. "Originally the thing I was so committed to and really wanted to get was the big baby quality of Orson in that moment," he says. "Not to be derisive, but he looked like a giant infant. He's fleshy and his features are very rounded. He had a rounded nose and rounded cheekbones. At 24, he seemed like if you put him in diapers and had him show up on the soundstage, he would look like a big baby.

"There was this sense of a child pulling the levers of this massive motion picture machine," Fincher adds. "As he said, 'A movie studio is the best train set a boy could ever have.' I really wanted to get that idea of this big kid."

That all changed when Fincher found Tom Burke, who plays Welles in the film. Throughout Mank, Welles often appears in shadows or under the brim of an oversize hat, sporting one of his signature capes. But it's his voice that is unmistakable.

"Tom's not a big kid, Tom's a man, and he has certain physical similarities," Fincher says. "But the thing that I kept coming back to was his voice was beautiful. His voice is very hypnotic, and we gave him recordings of Welles and encouraged him. Tom had the power. Tom had the gravitas. Tom had the presence, and he could definitely develop the voice."

Fincher says that by the end of filming, he would hear Burke over the phone or in voice-over and swear he was listening to Welles himself. It was that impeccable replication that helped Fincher find the ending of the film, an epilogue of Mank and Welles winning Oscars for writing Citizen Kane. Welles' speech is conveyed through voice-over, as if he were on a radio broadcast. But that wasn't the original plan.

"It was supposed to be Welles' sign-off in the movie, where he's got a Cuba libre in his hand and he's with all these showgirls in Brazil," Fincher says. "It was funny in a bad way. But what was amazing was when we played the audio from it, it was like, 'Is that a clip of the actual Orson Welles?'" Thus the final, more emotional version of the scene was born.

Welles has been gone since 1985, and yet he's still finding ways to change the final cut.

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