A brilliant actor and a highly educated man, Theo Marcuse was a favorite of mine in the days when secret agent type shows were all the rage. His early death ended what looked to be shaping up to be a brilliant career. Thanks for mentioning him,
Another superb actor, with excellent theater credentials, David J. Stewart, whose best film role was in
Murder, Inc.
(1960), also appeared in many classic TV shows. He never became a big name player due to his unusual looks,like an American Christiopher Lee with a bad complexionplus the impression he gave that he thought he was ugly, which made him look worse than, well, he looked, didn't help matters. Like Marcuse, Stewart died fairly young.

he died in a car crash a few weeks after catspaw

I love his wittily eloquent THRUSH chief on UNCLE and prison governor on THE WILD WILD WEST.

Wow. surprised to see somebody else taking notice of this fine actor (even if it's been a few months).
I remembered him from his role in
Star Trek
, but never really thought much about him until I watched some classic shows on DVD the other year and saw him in an episode of Karloff's
Thriller
and a number of
Man from UNCLE
episodes. Then I looked him up on IMDB, and saw that his life was indeed cut tragically short. Like you, I later started seeing him pop up all over METV.
The guy was definitely a terrific character actor who brought a unique and indefinable presence to his roles. It's a shame he didn't live to the 1970s - he would have made a great lead, or at the very least it would have been wonderful to see him popping up in guest roles for a couple more decades.
Also worth noting he was a highly-decorated US Navy officer during WWII for his service on the submarine
Tirante
.