Well, real hair of the proper color and texture is pressed into the slightly warmed wax with a scalpel one hair at a time. Each wave and curl of the subject's own hair is reproduced.
14 seconds sound clip from the House of Wax (1953) movie soundboard.
You can hear this line at 00:05:34 in the Blu-ray version of the movie.
Quote context
[...]
- You'll recall that Antony, believing Cleopatra to be dead, killed himself with his own sword.
- When Cleopatra discovered what happened, she quickly followed her lover.
- You noticed how cleverly the hair is mounted on the scalp?
- Yes. How do you do that, professor?
- That's the best figure of Booth I've seen. You almost expect him to speak.
- I wonder what he'd say after all these years. I'm sure he'd rant a bit.
[...]