Five-year-old Ronnie Howard (“Opie Taylor,” “Richie Cunningham”) in “Walking Distance” on Twilight Zone (S1, E5).
David Wayne (“The Mad Hatter,” “‘Digger’ Barnes”), Raymond Bailey (“Milburn Drysdale”), Virginia Christine (“Mrs Olsen”), Dick Wilson (“Mr Whipple”), and Joe Flynn (“Captain Binghampton”) in “Escape Clause” on Twilight Zone (S1, E6).
Maybe not a familiar actor but the ambulance driver from Silence of the Lambs shown below is Chicago radio personality Buzz Kilman
Troughton also was in “the Omen.” He played the doomed priest who tries to warn Gregory Peck that his adopted son is the spawn of Satan. Then he gets speared through the chest by a church spire.
Other “Who” sightings:
William Hartnell appears briefly in “Odd Man Out” as a bartender.
Jon Pertwee had a bit part in the movie version of “A Funny Thing Happened on the Way to the Forum”.
Tom Baker played “Rasputin” in “Nicholas and Alexandra.”
Christopher Ecclestone and Peter Capaldi were suspects in two different episodes of the “Hercule Poirot” series (with David Suchet).
Lalla Ward (Romana #2 and very briefly Mrs. Tom Baker) was a vampire in the early 1970s Hammer horror movie “Vampire Circus” (which is not a bad 70s horror flick, it’s actually worth seeking out.)
Louise Jameson (Leela) appeared on the British soap opera “EastEnders”.
Peter Capaldi also played Valmont’s valet in Dangerous Liaisons and a choir director in an episode of Midsomer Murders.
So was Sean Pertwee, which doesn’t count but should. 
Plenty of famous faces pop up in Suchet’s POIROT outings: you can, f’rinstance, see young Michael Fassbender, and, well, he’s instantly recognizable. The only one that threw me was the low-key guy who – wait, what the hell is wrong with his mouth? Sometimes it looks almost normal, but then it’ll look like a puckered scar in need of medical treatment. Heck, he almost looks like – Damian Lewis.
One that really surprised me - the guy who played “Elias” on Person of Interest (a character who runs a highly effective criminal underground, and is well-educated and highly intelligent) is the same guy who plays Mathesar, the leader of the Thermians in “Galaxy Quest.”
Jeannetta Arnette has quite an impressive list of TV show credits. Her appearance in the Criminal Minds episode “Riding the Lightning” was absolute perfection.
Also seen on Midsomer Murders:
Judy Cornwell who played Daisy on* Keeping Up Appearances*
Stephanie Cole who played Diana Trent on Waiting for God
Janine Duvitski who played Jane on Waiting for God
I watched Midsomer Murders for several years before I realized Barnaby’s wife was played by the same actress (Jane Wymark) who played Morwenna Chynoweth in the original Poldark:
https://tse4.mm.bing.net/th?id=OIP.kgOIjeZbLKl1k8zUOuhVYwEDDC&pid=Api
I have an odd one. A character actor by the name of Ian Wolfe who finally rose to notice on WKRP as the elderly butler Hirsch for Mrs. Carlson in 1981. I also knew him from many small roles in other 70s sitcoms like Barney Miller and at least 1 Star Trek episode as Mr Atoz.
So I’m watching a Lionel Barrymore movie from the 30s and there is this older guy in it that is the spitting image of Ian Wolfe, though maybe a little younger. I wonder if it is his father, sort like it is easy to confused Alan Hale & Alan Hale Jr.
But no, it turns out Ian Wolfe was already playing older characters in the 30s, I couldn’t believe it was the same actor.
Looks like the movies was On Borrowed Time (1939)
Even earlier ('35), “Mutiny on the Bounty,” with Gable and Laughton.
Wolfe actually had two classic Star Trek credits. He was in the Planet of the Romans episode, too: Ian Wolfe | Memory Alpha | Fandom. Died at age 95!
Not to blow my own horn too much, but did no one notice that I mentioned Ian Wolfe in post #240? 
But that was thirteen whole posts ago! 
Christopher Eccleston (Dr. Who) had the lead in the film Jude.
David Tennant (Dr. Who) has a walk on bit as a “drunken graduate student”.
No, I missed it, it is a very long thread and I thought he was obscure enough that I didn’t check very well.
No problem. It is getting to be a pretty long thread, and I suppose I can’t expect everyone to follow my every word with rapt attention.
Although it wouldn’t kill you all if you did! 
^ Once you’re back at the Library and head back in time, we won’t know who you are, anyway.
And there’s your explanation for how Wolfe was able to play little old men even back in the 1930s. Time travel! It’s so obvious!
Don’t know why we never thought of that before.
