Of all of TOS's villiains, he and Chekov's shooter from SPECTRE OF THE GUN had the scariest stares.
But as good character actors can, he could play roles so differently that most might not recognize him from part to part. There was an episode of HILL STREET BLUES in which Ruskin was slammed against a wall by one of the show's relatively wimpier characters (played by Joe Spano). It was years before I realized it was also Ruskin. Taken out of context, it's funny: Galt from Triskelion got wall-slammed by little Henry Goldblume.