I noticed several of my favorite TV shows have shared quite a few cast members. I guess it’s because they shared a writer and\or creator.
Like The Shield*Sons of Anarchy* (both written by the Kurt Sutter) where actors in recurring roles in both were Kurt Sutter, CCH Pounder, Walton Goggins, Jay Karnes, Benito Martinez, and Kenny Johnson.
Likewise, Sports Night*The West Wing* (both written by Aaron Sorkin) which shared Felicity Huffman, Joshua Malina, Timothy Davis-Reed, Ron Ostrow, Ted McGinley, Teri Polo, Clark Gregg, Lisa Edelstein and Jeff Mooring. (We can’t count Nina Siesmaszko and Janel Moloney, as they had only 1 episode each in Sports Night)
In the early days of FOX-TV, the same group of actors would end up in very similar roles in sitcoms. I can’t remember any names or titles without looking them up, so they obviously didn’t make much of an impression. But the sitcoms were all basic couples and groups type things.
Oz and Law and Order shared a lot of cast members. I think mostly because they were the only two ensemble shows that filmed in and around NYC at the time.
Cast members from Petticoat Junction and Green Acres also crossed over on occasion, most notably Frank Cady as Sam Drucker. This was natural, since they were set in the same fictional locale (Hooterville).
I’m not sure if you want to count this but Viva Variety had Kerri Kenny, Thomas Lennon and Michael Ian Black, all of which came from The State. The only reason I say you might not want to count it is because I believe those three may have been part of a troupe beforehand and VV was a spinoff of The State. Having said that, Kerri and Thomas went on to create and start in Reno 911! afterwards.
If you do want to count that first example, in the early days of SNL many of their people were plucked from SCTV (or Second City in general). But I probably wouldn’t count that. It’s like asking how many actors were in theater in high school or how many major league [sport] played in college.
From the black and white days, William Schallert and Jean Byron played Dobie’s teachers on The Many Loves of Dobie Gillis. A couple of years later they were Patty’s parents on The Patty Duke Show.
Around the same time, Warner Brothers produced a bunch of detective shows that had similar formats. So similar, in fact, that when Bourbon Street Beat was canceled, Richard Long’s character was transplanted to 77 Sunset Strip and Van Williams’ character went to Surfside Six.
Tim Conway and Joe Flynn, who had been foils in McHale’s Navy, were teamed up as partners a few years later in The Tim Conway Show.
The Closer / Major Crimes on TNT. It’s [almost] the exact same series, with only the principal character changing when Kyra Segdwick left the former and was replaced by Mary McDonnell in the latter.
I don’t think the OP means characters from one series popping up in another. There might be old episodes of Gunsmoke or Wagon Train, for instance, that have William Shatner and DeForest Kelley working together years before they appeared on Star Trek (I think that actually happened, but can’t think of specific instances).
When Mary Tyler Moore had her own production company, she drew heavily on actors who had worked for Second City and Compass, live improv and sketch companies that launched careers for Valerie Harper, Peter Bonerz and Roger Bowen among many others, and these actors showed up in different permutations in The Bob Newhart Show, Rhoda, and numerous lesser-known MTM productions. Norman Lear and (as previously noted) Aaron Sorkin have drawn heavily and repeatedly from small pools of actors.
Some married couples have turned up in lots of TV shows as paired items, like Richard Benjamin/Paula Prentiss, Stiller and Meara, Jill Eikenberry/Michael Tucker. Some comedy pairings like Burns & Schreiber, Penn & Teller, and Cheech & Chong have done so as well.
Some FRIENDS pairings have turned up post-FRIENDS; Jennifer Aniston turned up on one or two episodes of Dirt (starring Courtney Cox) and possibly Cox’s Cougar Town as well (or was that Lisa Kudrow?).
Nurse Jackie and Sopranos have Edie Falco and Paul Schulze. In Nurse Jackie, Edie is having an affair with Paul. In the Sopranos, Paul is the priest that Carmella…I’m not going to spoil anything for people that haven’t watched it.
Bill Cosby and Phylicia Rashad starred together in two different shows. Fred Gwynne and Al Lewis were in Car 54, Where Are You? as officers Muldoon and Schnauzer, then on The Munsters as Herman and Grandpa.