-Gyllenhaal is a very talented actor, and has experience playing mentally-ill characters (read: Nightcrawler, Donnie Darko, Enemy, etc.).
-His filmography has an excellent mix of dramatic and action-based roles.
-He was raised in a Jewish household, but is no longer practicing. Marc Spector is similarly a lapsed Jew.
-See Enemy, where he plays both a man and his doppelganger. Critics praised his ability to play two similar roles while still making each character subtly unique. He’d be a near-perfect choice to play Marc Spector and his multiple personalities.
”Did you like question ten, Moony?” asked Sirius as they emerged into the Entrance Hall.
“Loved it, said Lupin briskly. “Give five signs that identify the werewolf. Excellent question.”
“D'you think you managed to get all the signs?” said James in tones of mock concern.
”Тhink I did,” said Lupin seriously, as they joined the crowd thronging around the front doors eager to get out into the sunlit grounds. “One: he’s sitting on my chair. Two: he’s wearing my clothes. Three: his name’s Remus Lupin.”
“Josie had given up on elaborate hairstyles and excessive jewelry. She’d abandoned heavy makeup and showy gowns. Looking at her now—hair slightly curled, the hint of gloss on her lips, and an age-appropriate blue dress—it seemed she was finally stepping into her shoes instead of mine.”