Mark Hamill appears again, this time as Earth 3's incarnation of The Trickster, as an enemy of Jay Garrick (played here by John Wesley Shipp). This is a reference to the original series The Flash (1990) where Shipp starred as Barry Allen and Hamill originally appeared as his enemy, The Trickster.
Julian Albert, portrayed by Tom Felton, reveals the origin of Dr. Alchemy's power is the Philosopher's Stone, confirming that Felton's casting was, in itself, a hint. The same legendary object is the focus of Harry Potter and the Sorcerer's Stone (2001), where Felton co-starred as the bully Draco Malfoy.
Julian's full name is revealed to be Julian Albert Desmond. In the comics, Dr. Alchemy's real name is Albert Desmond.
The Trickster, played by Mark Hamill, refers to Jay Garrick as Swifty. Hamill was the voice of The Joker in Batman: The Animated Series (1992) (and several other shows) and uses the same recognizable voice as The Trickster. His Joker would also frequently refer to Batman as Batsy.
When on-set photos of Mark Hamill as the Earth-3 incarnation of the Trickster first appeared online, a couple of months before broadcast, fans speculated that he might be playing a version of the Joker instead.