John Barrowman has an extensive background in musical theater and now he gets to bring that to the Arrowverse. Barrowman will be appearing in the musical crossover episode of The Flash. His character of Malcolm Meryln first appeared in season one of Arrow and has been around at least some in every season since until this year where he has been part of the Legion of Doom on DC's Legends of Tomorrow. This is his first appearance on The Flash, his was one of the first contracts to be to the studio and not to the individual show, allowing for crossovers like this to take place easily.
The Flash airs Tuesdays at 8 PM on the CW.