Watch 'American Idol' Alum Justin Guarini Take on an Alter Ego in Diet Dr Pepper Ad

Justin Guarini Dr Pepper ad

American Idol season one runner-up Justin Guarini stars in a new ad campaign for Diet Dr Pepper.

Guarini takes on an alter ego named Lil' Sweet in the commercial. The pint-sized heavy metal-soul man shows up at a suburban home unexpectedly -- where a child's birthday party is in progress -- to deliver “zero calorie sweet treats.”

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“He sings in the key of sweet and always carries a spare six pack of Diet Dr Pepper with him,” reads a description on the soft drink's YouTube channel. “No one knows where he keeps it though.”

In the ad, Lil Sweet defies gravity by sliding down the banister as he rescues two dads craving a cupcake, but fearful of the calories.

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In other news, Guarini recently appeared as Julio in a revival of Paint Your Wagon at New York City Center. He is also set to star as Bobby in the play Company at the Bucks County Playhouse from May 29 through June 21.

His Idol cohort Kelly Clarkson is slated to appear on the Fox show as a mentor on April 1.

Watch the commercial below: