Caught in the Act

Gerard Butler Paints a Sexy Mural at Coachella Party

04/17/2012 at 05:25 AM EDT

Gerard Butler braved the freezing cold and headed to the GenArt MUSE afterparty during the Coachella Valley Music and Arts Festival in Indio, Calif. – and he had artistic ambitions.

Arriving at the Ciroc vodka-sponsored soiree with a group of pals, the actor excitedly jumped at the opportunity to contribute to the Paul Frank mural that featured many intricate designs by other various partygoers. He painted a portion of the wall, which will be donated to GenArt.

"He selected what he called the "sexiest" part of the mural to sign," an onlooker tells PEOPLE. "It featured a woman's leg as the guitar strings."

NEXT: Vanessa Hudgens Shares a Vodka-Flavored Snow Cone with Boyfriend