It wasn't until Trek VI that movie-actors entered the picture, via Christopher Plummer and David Warner.
Sorry, I gotta take exception to this. First, Brock Peters had a huge career in films long before the Trek movies and he debuted as Admiral Cartwright in Trek 4. Paul Winfield also had a nice career in films before his turn in Trek 2.
But it's the whole "movie actor" thing that bugs me. And it's not just you, it's a stigma, but it's unfair (so please don't take this the wrong way).
Actors are actors. It's box office that measures a actor's success in the world of feature films (yet Jennifer Aniston keeps getting leads even though most of her films bomb - weird). JJ Abrams, for example, populates his featured roles with actors who have worked with him on TV and there's nothing in the TV alum's performance that tags them as TV actors. George Clooney, the quintessential movie star today, was a regular on The Facts of Life and a hit on E.R. Was HE a "TV actor"? What changed about his work that altered his status? He's the same guy whether he's playing Dr. Doug Ross or Danny Ocean.
Shatner, Nimoy and the rest aren't "TV actors" but "actors." De Kelley had a long career in films pre-Trek. I would never consider him a "TV actor." They were all pros who had done films, yet simply had their greatest success on the small screen.