Just watched an old made-for-TV movie from 1977 called “Ants”. Among the cast were Robert Foxworth and Bernie Casey. The former portrayed Admiral Leyton, the latter Calvin Hudson, both former comrades of Benjamin Sisko, both of whom betrayed Starfleet.
Also in the cast was Barry VanDyke, who later had his own space adventures on Galactica 1980.
I’ve seen that, it is pretty bad.
I’ve mentioned watching Dr. Kildare episodes from the early 60s and upthread I cited one with Nimoy in it. Last night we started a 5-parter (each a half hour, from the last season).
It has Shatner in it as a “heartless” doctor. But he actually gets easily angered so not doing Spock. Listed as a special guest star.
As a bonus it also has the great Martin Balsam as a GP/patient. Balsam’s wife is portrayed by Diana Muldaur. She appeared twice in TOS in different roles. The episode Return to Tomorrow has her as a doctor who gets possessed by an ancient alien so sort of a double role. She was also a different doctor as a regular on the 2nd season of TNG. She’s still alive at 86.
But wait, there’s more.
There’s a lazy intern character played by Bruce Hyde who was in two first season episodes of TOS as “Riley”. He sang an Irish song in one episode. Unlike the above, his career was quite short. Appeared in only 8 things.
Possibly Diana Muldaur’s most famous line is “Can Lorca ride?” which is from Gunsmoke episode Fandango. Most of the people who hear it will not actually recognize her voice: it is in interstitial background chatter on Pink Floyd’s The Wall.
Just saw Erick Avari in an old West Wing, playing the Pakistani ambassador to the U.S.; he played three different ST roles.
Michael McKean appears as an ambitious Customs and Immigration agent in Coneheads (1993), of which I just saw a little.
Watched Novocaine starring Jack Quaid. Pretty stupid overall.
He left show business to become a hippie, tho he soon went back into conventional society as a teacher and fine arts director.
Just saw Paul Winfield in the courtroom drama Presumed Innocent (1990), as a no-nonsense judge.
Tucker Smallwood also had a small role as a homicide detective.
I saw Brent Spiner in the 1997 Broadway revival of 1776, playing the lead role, John Adams. He was much more forceful and full-voiced than I expected. Here’s a clip of the opening number; Spiner delivers the opening lines around 2:00 and sings around 3:15: https://www.youtube.com/watch?v=DVrQrm3Pznw
Likely mentioned upthread (and years ago), but we were watching some season 1 episodes of Leverage on Ovation this afternoon, and the episode “The Juror #6 Job” featured Brent Spiner as the CEO of an herbal-supplement company who is on trial for the death of someone who took his supplements, and Armin Shimerman as a doctor who is brought in to testify as a witness in his defense. In addition, the episode was directed by Jonathan Frakes.
Just saw Marina Sirtis in a 1986 episode of Granada TV’s Sherlock Holmes series, starring Jeremy Brett; she plays a London Mafia don’s daughter. That was just a year before ST:TNG premiered. The whole episode (“The Six Napoleons”) is worth watching, but you can see her at 1:29, 5:32 and 49:07 here: https://www.youtube.com/watch?v=8IYBuV9Q5Bs
Wil Wheaton appears as “Chaos” the mad hacker in at least two episodes of Leverage, and Jeri Ryan subs for one of the main characters in several episodes when that character goes home for a while. But, in the sequel series Leverage: Redemption, possibly the most memorable episode features Levar Burton.
I had forgotten about that! Ryan is in the second half of Season 2; the in-character explanation was that Sophie (Gina Bellman) had taken a leave from the team, and had recommended a friend of hers, Tara (Ryan) to fill in for her. The out-of-character explanation was that Bellman was pregnant. 
Just saw the 1962 Frank Sinatra political thriller The Manchurian Candidate over the weekend. Whit Bissell plays a senior Army officer, and James Gregory plays a blowhard, McCarthyesque U.S. senator.
Whit Bissell again in the 1948 thriller He Walked by Night. His voice is unmistakable, but it took me a second to recognize his face. Also in the cast were Richard Basehart, Jack Webb, John Dehner, Kenneth Tobey, and Frank Cady.
I was watching Columbo’s “Publish or Perish” tonight when I spotted James B Sikking as a cop at the scene of the crime. He was in it so briefly that he’s listed as “Uncredited,” but he’s another actor whose voice is unmistakable.
Mariette Hartley is in this episode too, the first of the pair in which she guest starred.
James Gregory is Ursus in Beneath The Planet of the Apes.
“The only good human… is a dead human!”
Recently saw Sam Anderson as a bad guy on NCIS.
A future ST:TNG actor and actress appeared in the same 1984 Granada TV adaptation of the Sherlock Holmes tale “The Speckled Band,” playing an abusive country squire and his frightened stepdaughter. I just watched it last night.