Boris Karloff's voice changes when he speaks for the Grinch. Originally he spoke in his "Narrator" voice throughout. After recording was complete, the highs in his voice were mechanically removed for the Grinch, giving him the gravelly voice heard in the finished version.
Thurl Ravenscroft, who sang the song, is best known as the voice of Tony the Tiger in numerous Frosted Flakes TV commercials. Ravenscroft was picked to sing these songs because his voice was an extremely deep bass.
Though all of the production and character designs were based upon original artwork from the book, Dr. Seuss thought that the Grinch more closely resembled Chuck Jones rather than the original Grinch drawings.
The lyrics to the song "Fahoo Foraze" were made to imitate classical Latin. After the special aired, the studio received letters asking for a translation from people who believed them to be real Latin.