Tom Hiddleston performed all his own vocals. During the concert sequences he sings to his own pre-recorded tracks, but on the numbers "I Saw the Light" and "Your Cheatin' Heart" he sings live on camera.
In real life, Toby Marshall bought a bogus medical degree for $25. He lived in Oklahoma City and connected with Hank Williams in 1952, becoming his "personal physician." He prescribed chloral hydrate to treat alcoholism. After Hank died, Lilly summoned Toby for advice, which he promptly billed her for. Toby was eventually investigated in connection with Hank's death, and imprisoned after his wife died from similar treatment. He died in Oklahoma State Prison in McAlester, Ok.
During filming, Tom Hiddleston endeared himself to the cast and crew by serenading them at the piano between scenes. At wrap, each day, he graciously shook hands with everyone involved and thanked them for their work, from the janitor to his costars.
His clear musical talent and unpretentious behavior on set quelled any suspicions that the Englishman wasn't right for the part.
To gain his pencil-thin appearance, Tom Hiddleston ran ten miles and cycled 15 miles every day.