Oh yea!
This reminds me of the famously interchangeable Chris’ from a few years ago. They were all in superhero movies all fairly similar.
As for the Ryans- Jake Gyllenhaal should have been named Ryan also. I have to hear the voice to tell him from Gosling.
I have never been able to tell Tillys apart.
I used to get actor Guy Pearce and director Guy Ritchie conflated, and genuinely thought they were the same Guy. I thought he was a ridiculously talented actor-director, although much better as a director than an actor.
I’ve mentioned before in a similar thread that I seriously cannot tell Tony Amendola apart from F. Murray Abraham. I have to watch the credits to figure it out. For quite a while, I seriously didn’t even realize they were two different people, and thought F. Murray Abraham had a recurring role on Stargate: SG-1.
Tony Amendola does a lot of sci-fi/fantasy TV, so if it’s a sci-fi/fantasy or sci-fi/fantasy adjacent show, I can be pretty confident it’s him. MythicQuest got me there, though - that one actually is F. Murray Abraham.
For the longest time I had no idea Hugo Weaving was not in fact Dennis Quaid
I’ve mentioned this before, but in the pilot episode for “NewsRadio” the character that later became Joe Rogan’s character in the regular series was played by an actor named Greg Lee. For years I was absolutely positive that it was Enrico Colantoni from “Just Shoot Me”. (Also I can never remember if it is “Colantoni” or “Colantino”)
Speaking of Hugo Weaving, sometimes I get him and Sam Neill mixed up.
I used to think Enrico Colantino was Joey Pants. I didn’t know of an Enrico Colantino but, dang, Joey Pantoliano can play anything!
I could have sworn George R. R. Martin had a walk-on role in Fargo:
Wait, Sam Neill and Dennis Quaid are different people ? 
Watching 1951’s Hoodlum Empire just now, I remembered I used to confuse these two guys:
and
I best remember Luther as “Dominick Vashon” on Hawaii Five-O (1972).
It took me a while to differentiate Michael Cera from Jesse Eisenberg. It wasn’t until I saw the credits that I realized Cera wasn’t in The Social Network.
I had heard (after the fact) that although Frasier’s dog “Eddie” was played by Moose, by the end of the series Moose’s son Enzo played the role. A couple of fun tidbits I found, reading up on them…
When Moose had to lick his co-stars, however, sardine oil was applied upon the actors’ faces. John Mahoney once revealed liver pâté was dabbed behind the actor’s ears to make Moose nuzzle the actors.[2]
Enzo:
Later in the series, he was used as a stunt double to perform the more physically challenging tricks for his aging father. Enzo and Moose took turns playing the role after the eighth season. Enzo was also used as one of the puppies that “Eddie” had fathered during the show. Off the set, trainer Mathilde de Cagny has stated that Moose and Enzo’s relationship was so bad that the two “could not stand to be in the same room together.”[2]
Maybe they picked up the friction between Frasier and Martin? Enzo also played Skip (in “My Dog Skip”).
A 1999 interview quotes the director of My Dog Skip :
“Skip never failed us. I wish I worked with actors who were as well prepared as Skip. There was not a trick or a piece of business we asked the dog to do that he wasn’t able to do; it was uncanny. The trainers were so good, they could stop him on a mark, he could lift his leg, he could do a somersault. I expected to see him reading The New York Times any day.”[3]
Jeez… Dennis Quaid. Not Randy. Your post makes so much more sense now. 
NITPICK: Eddie was Martin’s dog, not Frasier’s. Frasier couldn’t stand him.
Richard Johnson:
Eric Braedon:
I usually confuse Luther Adler with his brother Jay:
And when it comes to Olsen twins, I can’t tell Mary-Kate from Ashley (or Mary-Ashley from Kate…)
Right, sorry, brain fart. It was so great when Eddie would just fix him with a stare…
Close, but I think Gary Raymond is even closer:
Matt Damon
and Leonardo DiCaprio look alike to me.
That made the movie The Departed really confusing for me, since both of them were in it, and I kept getting their characters mixed up.
Remember Terrance Malick’s The Thin Red Line from 1998? Arguably, the film’s two main characters were played by two relatively unknown actors at the time:
Jim Caviezel:
and Adrian Brody:
When I saw the film for the first time, in the theaters, I didn’t just mix them up - I actually had no idea that they were two separate actors playing two separate characters. I only realized my error when the movie ended - Caviezel’s character had died, and Brody’s character had survived. Or maybe it was vice versa?
I used to confuse Lee Marvin with James Coburn. And, now that I look it up, I see that some fans thought that they were brothers, due to the similarity in appearance.
I always thought the spokes-guy in this commercial was an actor I had seen on some blue-collar comedy like King of Queens. But apparently not. His name is Rob Biesenbach and he has no TV show credits on IMDB. It’s that way he has of speaking with his lower lip and his jaw stuck out that seems so familiar.
