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The series, based on the memoirs of comedian Chelsea Handler, stars Laura Prepon as an opinionated cocktail waitress.
Cassidy has now signed up to play a love interest for Prepon's character Chelsea, TV Line reports.
His character, a Harvard graduate, is said to be a good-looking, confident redhead who is knowledgeable about IT.
The role was originally played in the pilot by Jake McDorman, but he has since been promoted to a series regular and will now play a bartender in the comedy.
Cassidy has previously starred in shows such as Privileged, The OC and Smallville.
Ali Wong also recently signed up for a role in Are You There, Vodka? It's Me, Chelsea, which will premiere on NBC in midseason.
Watch a preview of Are You There, Vodka? It's Me, Chelsea below:
