What I find weird...
First the star, Paul Massie, to me looks like a dead ringer for one of those Neanderthals in the insurance commercials when he's in his Dr. Jekyll makeup. It makes it hard for me to take him seriously. I do like the creativity involved in making Jekyll the made up one and Hyde the normal looking actor.
Then, the manipulation of Jekyll's voice. Again, it seems the actor's normal voice is Hyde's, perhaps speaking a bit higher and lighter than normal for increased contrast. In some way Jekyll's is made to sound deeper, though whatever they did sounds annoying to me somehow that I can't identify. But what was the method used?
Nowadays this is no problem to do digitally, and I know there were commercially available electronic methods for speeding up or slowing down a voice without changing its pitch in the 1980's, which I believe implies the reverse could be done, i.e. changing the pitch without changing the speed. But back in 1960? I remember the cartoon "The Chipmunks" but I've always assumed the voices were prerecorded speaking very slowly then played back at higher speed to raise the pitch AND speed, making the pitch high and the speed normal.
If the reverse of this was done in this movie to make the speed of speaking normal but the pitch of his voice low, they did some awfully fine work getting it just right and dubbing it perfectly. Too perfectly it seems to me. Which is why I have another theory:
Everyone knows you can inhale helium (which is less dense than air) to make your voice sound high because your vocal cords vibrate faster in the less dense environment of helium. I only learned relatively recently something that probably should have been obvious: there are denser-than-air gases that can safely do the reverse - make your voice sound much lower. This is what I'm thinking they did in this case - have the actor take a hit off a tank of air mixed with this gas (that I don't recall the name of) then have him deliver his lines for the camera.
I'm of course assuming that the actor couldn't manipulate his own voice to this degree naturally. It seems far too extreme a change for it to be done unassisted in some way.
Anyone have any other guesses or have read anything about how it was accomplished in the movie?