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.
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.
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 received no screen credit for his singing, an oversight Dr. Seuss attempted to rectify by sending letters to every major columnist in America identifying Ravenscroft as the singer on "You're a Mean One, Mr. Grinch". He is also part of the chorus on the other two songs.
Throughout the movie, the Grinch's eye color is red, indicting his bitter, misanthropic and sneaky personality. But towards the end, where he discovers the true meaning of Christmas by the sight of the still happy rejoicing Whos in the deprived town of Whoville, his eyes turn blue; meaning that he has a change of heart.