Knepper will play DC Comics villain William "Clock King" Tockman, a brilliant criminal who prides himself on his precision. He'll appear in the 14th episode of the season.
Knepper most recently starred as an enigmatic actor playing a serial killer on the short-lived CW series Cult. Prior to that he starred on Prison Break and is currently appearing on the TNT series Mob City.
Arrow's fall finale airs Wednesday at 8/7c on The CW.
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