I call the franchise the "Dick Wolf Casting Agency" because - like with Raul Esparza, who did a guest role here then ended up as a regular SVU cast member - the guest roles seem to be a pool for Dick Wolf to use for future casting. As discussed in the Mothership thread, I saw the actor that played Eames' temporary partner in "Purgatory", when Bobby went undercover without Eames' knowledge, as a defense attorney.
Then there was the late great Jerry Orbach as defense attorney Frank Lehrman also on the Mothership in the S2 episode, "Wages of Love" (based on Betty Broderick), before popping up a season later as Lennie Briscoe.
And S. Epatha Merkerson was in a S1 Original Recipe Show episode, "Mushrooms", as a woman whose baby gets shot to death. She naturally reemerged in S4 as Anita Van Buren.
Jeremy Sisto followed Jerry Orbach's path of casting...
And Julianne Nicholson was on the one and done season of the 2006 show, Conviction (not to be confused with the more recent failed show of the same name with Hayley Atwell!), a show about young ADAs in the L&O universe. Once that show ended, she went to CI as Det. Megan Wheeler and Milena Govich went to the Mothership. Wheeler took fine; Cassady, Govich's character, left after a single season.
And Kelli Giddish guest-starred in the S7 CI episode, "Depths" (ironically on early tomorrow morning at 3:00 a.m. ET per listings on Oxygen) before going to SVU as hot-mess Amanda Rollins. (Spoke about this before, but I find it odd that Giddish's SVU character has the same exact name as Cynthia Nixon's CI character, an alcoholic Broadway producer, in the S10 episode, "Icarus". That was in 2011, just months before Giddish joined SVU, too. Wonder if there is a relative or friend and that's why the name got used twice within months? LOL!)
But all of the above is why I love the "Dick Wolf Casting Agency". LOL! In a related note, he seems to reward/remember some former cast members of his other non-L&O shows with some guest roles. Patti D'Arbanville, who was a cast member on New York Undercover (Yep, Dick Wolf was a co-creator), guest starred on Original Recipe and Criminal Intent. (Unsure if she did the trifecta?) And Lauren Velez, who was also a cast member of New York Undercover, played dirty beat cop (after being demoted from detective) Lois Melago on CI in the episode, "Purgatory", who was the former partner of fellow dirty cop, Mike Stoat, who Bobby was tasked with bringing down. Played by none other than Mayhem himself, Dean Winters, better known from SVU as Cassidy.
So Raul Esparza was part of an esteemed group. 😛