Karloff's back issues...
One thing I noticed in this film is that Boris obviously has some pretty extreme back pain. If you watch carefully in some of the scenes he is walking across the castle in, you can see him kind of walking like an ape. I read somewhere that he had some pretty severe back problems, and this is one manifestation of that pain. I know because I have suffered from back pain for many years. I just had surgery last month to try and relieve some of my pain. It's interesting to me because I was walking the same way the last month or two before my surgery.
Just my two cents.