The Trickster poisons people at the Mayor's press conference that Iris is attending in episode 1x17 “Tricksters”.
Barry and Cisco go to meet with Lila to get the alien power source to power their Speed Force Bazooka but she turns them down in episode 3x22 “Infantino Street”.
Dr. Glassman joins Dr. Melendez to critique him for giving Shaun all of the scut work in episode 1x02 “Mount Rushmore”.
Alie and Jaha talk in the City of Light in episode 3x02 “Wanheda: Part 2”. Alie tells Jaha that Raven knows how to destroy her in episode 3x11 “Nevermore”. Jaha and Alie use Monty's mother to slow Raven's attack in episode 3x14 “Red Sky at Morning”.
Cassandra trains with some soldiers before being kidnapped by Snart and Rory in episode 1x13 “Leviathan”.
Source: VanFlarrowFan — Twitter
Neron talks to congress about the magical creature threat as Ray and then uses Zari to fake a monster attack to create fear in episode 4x16 “Hey, World!”.
Cyborg Superman breaks into the lobby of L Corp to steal the isotope needed to use the Medusa virus but is thwarted by Kara in episode 2x08 “Medusa”. Snowbird poses as Kara and gets into the elevator with Lena to meet her before being admonished by Eve in episode 4x16 “The House of L”. Kara expresses disgust at the DEO ad starring Lex, Lena, and her that she sees while out walking with Alex. Later, Kara talks to William about his distrust of Lex during Lex's press conference in episode 5x10 “The Bottle Episode”.
Source: What's Filming
Lyra takes Winn to the art gallery and convinces him that they should break in to have some hot museum sex in episode 2x16 “Star-Crossed”.
Lena and James both testify against Lex, he poisons the judge and the jury as the verdict is read, and meets Eve for the first time as he is being marched to prison in episode 4x16 “The House of L”.
Source: What's Filming — Twitter
Kiera arrives in the future at the feet of Kagame and is then greeted by future-Alec in episode 4x06 “Final Hour”.
