So what’s a nice guy like him doing with a bad-boy résumé like his? Blame that face. “The Cro-Magnon” look, he calls it — wide forehead, frostbit eyes, a chin so square it must make mathematicians swoon. Even before he was cast as Peter Quinn, the C.I.A. assassin he played for five seasons on “Homeland,” people would assume he’d come to kill them.

“It stopped them picking fights,” he said.

That face was a brilliant fit for Quinn, introduced in Season 2 as a coldhearted operative with a soft spot for Carrie Mathison, the show’s lead, played by Claire Danes. Alex Gansa, the showrunner for “Homeland,” said that he had cast Mr. Friend, then a relative unknown, because “he had these leading man looks and yet there were deep currents running underneath.”

Quinn was meant to die in Season 5, a victim of sarin gas poisoning. But when he learned that his character would perish, Mr. Friend was asked to draft the not-quite-a-love-letter that Quinn leaves Carrie from his deathbed: “Just think of me as a light on the headlands, a beacon, steering you clear of the rocks.”

“It was so beautifully composed and wonderful,” Mr. Gansa said, that the producers brought Quinn back for Season 6. “In a way he saved himself.”

The Quinn who returned was a damaged man, weakened by a stroke, tormented by his shortcomings. Fans were drawn to Mr. Friend’s mix of reserve and vulnerability, his portrait of a man soldiering on despite serious PTSD.

When Quinn finally met his end, taking a hail of bullets to protect the president-elect, some fans were so incensed that they bought a full-page ad in The Hollywood Reporter, shaming the producers for, among other things, Quinn’s incapacity “for loving and being loved.”