But, really, the point I set out to make is: Cassidy makes this role work because he doesn't look or sound like anyone else on the set. (He's the tall guy in this old photo. Anyone who's heard his deep resonant voice isn't likely to forget it--see/hear a multitude of examples at YouTube.) His special effect was who he was.
Likewise, the original Andorian on Star Trek was played by Reggie Nalder. He's quite plausible because, even without makeup, he exuded a certain inhuman malignant intelligence.
Character actors of this sort are a dying breed in modern Hollywood (if they aren't utterly extinct), and there is an oppressive sameness to modern casts. Everyone is about the same height. Everyone is about the same age. Everyone sounds very similar.
The result isn't necessarily bad casting, but it can easily become boring casting. If I