Mark Hamill plays the Trickster on the 90s Flash and reprises his roll on the CW Flash series as both the Trickster of Earth 1 and Earth 3, Earth-3 trickster in season 3 episode 9. There are a few references to the 90’s Flash in season 1 episode 17, Trickster.
The pictures in the newspaper about Jesse’s trial and conviction are screenshots from Mark Hamill’s appearance in “The Flash: Trial of the Trickster” episode of the 90s series.
The music for this episode includes Shirley Walker’s theme for the Trickster from The Flash (1990).
When Mayor Anthony Bellows (Vito D’Ambrosio) asks The Trickster (Mark Hamill) who he is, he replies “You forgot me already?” This is a reference to The Flash (1990), in which both actors played the same characters.
The trench coat Henry Allen (John Wesley Shipp) wears at the end of the episode, before Joe takes him back to Iron Heights Prison, is the same jacket he wore several times as Barry in The Flash (1990). According to John Wesley Shipp in a DC Daily interview, this came about because he mentioned that he still had the outfit and was told he has to wear it.
Vito D’Ambrosio To Reprise His Role as The Flash’s Tony Bellows From 1990 Series. In the 90s show he was a police officer; on the CW show he’s the Mayor of Central City. The IMDB link also sites this.
Alex Desert reprised his role as Julio Mendez in season 3 of the CW Flash.
Corinne Bohrer reprises her role as Zoey Clark, Prank, Tricker’s sidekick, in the CW Flash episode The Elongated Knight Rises.
The Flash Brings Back Corinne Bohrer From 1990 Series As Prank.
CW Flash season 6 episode 9 spoiler Crisis on Infinite Earths crossover
In season 5 episode 8 of the CW Flash the 90s Flash is revealed to live on Earth-90. In the Crisis on Infinite Earths Crossover the Earth-90 Flash, played by John Wesley Shipp, who played Barry Allen on the 90s show, steals the CW’s Flash speed. “I’m momentarily stealing your speed. You play this speedster game long enough, you learn some neat tricks.” He sacrifices himself to save the world instead of the CW Barry Allen. In his final moments the 90s Flash theme plays and a clip from the 90s show plays in wide screen. The 90s show was not wide screen. 13:36 in the crisis aftermath mentions having to change the aspect ratio The flash season 6 episode 9 | Earth 90 flash dies - YouTube 'This was a send off for the 90s Flash who never got a proper finale for the show. 15:29 of the same Aftermath show it’s mentioned the Shipp was grateful to be able to close that chapter.
Initially, CBS didn’t want Barry Allen to HAVE a costume, just a gray sweatsuit.
The Trickster was the only villain on the show who wore a costume.
If season 2 of the show were made, the Rouges, Captain Cold, the Mirror Master and the Trickster, would have teamed up to try and take down the Flash.
two sources for this fact, a link and book: The Flash: The Fastest Show On Television
Hofius, Jason; George Khoury (2010).
Age Of TV Heroes: The Live-Action Adventures Of Your Favorite Comic Book Characters* . TwoMorrows Publishing. ISBN 1-60549-010-5.
The series featured the first live action appearence of S.T.A.R. Labs.
When Christina McGee calls Barry for the first time, she tells him to come to “50, Garrick Ave”. Jay Garrick was the name of the first Flash, and the series premiered in the year of his fiftieth anniversary of publication.
The series is replete with references to the comic books and their creators. In addition to the “Garrick Ave.” reference mentioned in another item, episodes have referenced “The Hotel Infantino” (a nod to Silver Age Flash co-creator Carmine Infantino), “police captain Julius Schwartz” (a nod to Silver Age Flash co-creator Julius Schwartz), “the intersection of Gardner and Fox” (a nod to Golden Age Flash creator Gardner Fox), “Professor Zoom” (a recurring villain in the Silver Age Flash stories) and “Gorilla Grodd” (another recurring Silver Age Flash villain).
According to producers, had the series received a second season it would have featured appearances by classic Flash villains Vandal Savage and Weather Wizard.
In the DC Daily interview, John Wesley Shipp reveales it took a whole night of filming, “from dusk to dawn,” to figure out how to film the super speed shots. He had to run a certain way to not cause him to bounce up and down when sped up. In the end they told him to just try to run as smooth as possible.
He also mentions in the interview, when he saw Mark Hamill work with the Trickster suit, he knew if Hamill can do it then he needs get over his self consciousness of being in the superhero suit, not being comfortable in it and not wanting to talk in it.
“The  Flash is the most expensive weekly series in television history [(possibly at the time)], budgeted at $1.6 million per episode[.]” You think they wouldn’t have budget problems with money like that, “but instead, [show creators] Paul and Danny frequently seemed concerned over every cost.”
From the beginning of the “How it works on TV” section of “The All-New Flash TV Special”