While his body of work isn’t of a par with the other actors mentioned here, like Nielsen and DeNiro, I’ll give a call-out to Chris Hemsworth.
Up until 2015, his roles were nearly always on the serious side, or in straight-up action roles, particuarly his appearances as Thor in the first two Thor films, and the first two Avengers films.
Then, someone in Hollywood must have figured out that the guy has comedy skills – he started getting cast in more comedic roles (e.g., Vacation, Ghostbusters), and his later appearances as Thor (starting with the “Team Thor” video shorts, but particularly in Thor: Ragnarok) became substantially funnier.
I will say, it was really odd for me the first time I saw Forbidden Planet. “Wow, that guy looks sort of like Leslie Nielsen. And he sounds exactly like him!”.
On the female side, Jane Fonda started out doing frothy romcoms, then took a serious turn (and won two Academy Awards) and eventually worked her way back to older-woman romantic comedies.
Diane Keaton is more recognized for *Annie Hall *than she is for Reds.
The female Leslie Nielsen may be Candice Bergen. As a dramatic actress she was considered “stiff” with unfortunate comparisons to Charlie McCarthy. After dropping out of movies for several years, she scored big as a comic actress. Eventually she acknowledged she had picked up comic tips from Charlie McCarthy.
Joanna Lumley. She started off in 1970’s spy thriller roles like *New Avengers *and Sapphire and Steel. Then she did a spot on some comedy show - I forget what, now - as an oblivious twit wrapped round a bottle of wine, and was so hilarious at it that she got to reprise the role for about a decade as Patsy in Ab Fab
That’s what I heard too. Or to elaborate more, the brilliance of Airplane! is that they cast serious actors, put them in this ridiculous situation, and had them act like it was completely serious (and don’t call me Shirley).
ETA: And since Airplane! came out the year I was born, I pretty much only knew Leslie Nielsen for his comedies. I didn’t even realize he had been known as a serious actor until he died. And that makes me wonder if there’s some humor in Airplane that I’ve missed all these years that older people get.
Doesn’t really fit the OP - Reiser started as a standup before moving into acting, and his first major-ish role was in Beverly Hills Cop…Aliens was his second. So he’s always been in a combination of serious and comedic roles.
Man, that picture…it looks like a scene from a horror movie, just before Charlie pushes Candace down the stairs. (Also, she apparently really resented Charlie, since her dad gave ‘him’ more attention than her.)
There are a number of actors like Chuck Norris, who played serious roles so unconvincingly, people started to laugh at them. Maybe also Anthony Quinn, Charles Bronson. I think Lee Marvin started doing some light roles.
I fondly remember his role as Dr. Emilio Lizardo in The Adventures of Buckaroo Banzai Across the 8th Dimension, which is definitely comedic. That being said, he was better known at the time for his serious supporting role in Footloose earlier that year (1984), so that wouldn’t disqualify him for this thread.
I wanna say Martin Landau, but I haven’t seen enough of his films to be sure. I’ve seen him onscreen many times, of course, but I really started paying attention to him in Ed Wood. He was also in Crimes and Misdemeanors, and those two are his only (dark) comediic roles that I recognize, although his IMDB credits (again, the ones I recognize) suggest that he did a lot more comedy later in his career. Brilliant actor.
Harry Morgan had a long series of serious roles, including lots of cowboys and cops (I mean, Dragnet is as straitlaced as it gets), before joining MAS*H. After that he started picking up lighter roles in Disney films.
I think Charlie Sheen qualifies. Early on he was known mostly for heavy dramas like Platoon and Wall Street. Then starting with Major League he began doing comedies. The middle part of his career was a mix. His later career has been dominated with TV sitcoms.
*I’m not counting Ferris Bueller since that was little more than a cameo.