Regan first came on UK TV screens in series 5 (1992) of LWT's London's Burning, playing Don, the love interest of Firefighter Kate Stevens (Samantha Beckinsale). That same year he played PC Shelby alongside Sir David Jason’s DI Jack Frost in the (TV series) [A Touch of Frost]. Regan has over 15 stage roles to his credit, most recently as Achilles in Troilus and Cressida at the Edinburgh International Festival. In association with the Royal Shakespeare Company, the festival presented director Peter Stein's strongly cast and visually striking production. He is best known for Ancient Greece-based films 300, in which he played the Captain, Troy, where he played Eudorus, Achilles' second in command, and Clash of the Titans, in which he played King Cepheus. In 1999, Regan received an Irish Best Actor nomination for his co-starring performance in the award-winning miniseries Eureka Street. In 2006–07, he guest starred in two episodes of Wild at Heart as Simon Adams, father of Danny Trevannion's (Stephen Tompkinson) stepchildren Olivia and Evan. In 2012, Regan announced his intention to launch a theatre company in Beverley, Yorkshire, along with Judi Dench and other professional actors.[2] That year, he appeared in films such as Outside Bet, Lockout, and Snow White and the Huntsman. Regan is also a screenwriter and director, developing projects for his own film company. His script, Come Like Shadows, is an accessible but classic interpretation of Macbeth headlining Sean Bean and Tilda Swinton and will be directed by John Maybury. He also appeared briefly as a sexually confused character in British series The Street. In 2013, he guest starred in the first episode of Series 10 of New Tricks. He played the Duke of Savoy in the 4th episode of the BBC series The Musketeers in 2014. Regan joined the cast of the BBC's Atlantis as Dion for series 2 starting November 2014. He starred as King Simon in E!'s The Royals. Regan has acted in three films set in Ancient Greece: Troy, 300 and Clash of the Titans, and the Ancient Greece-based TV series Atlantis.

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