"You have to be smart to play a dumb blonde over and over and keep the audience's attention without extraordinary physical equipment."
Despite her image of a "dumb blond", she was actually very smart. Her I.Q was measured at 172. She often said that it took a lot of smarts to convince people that her characters were stupid.
According to her biographer Gary Carey, Holliday flummoxed the House Unamerican Activities Committee so much (by essentially playing her Billie Dawn character on the witness stand) she ended up being the only person ever called before HUAC who was neither blacklisted nor compelled to name names.