Gia Coppola tells all about Andrew Garfield’s ‘Mainstream’ fake penis

How do you get actor Andrew Garfield — Tony Award winner, former Spider-Man — to tap into his inner Johnny Knoxville?

Have the real guy on hand, said director Gia Coppola. The “Jackass” series creator plays a small role in Coppola’s new movie, “Mainstream,” out Friday, which stars Garfield as a charismatic drifter who becomes an internet sensation. For some of the film’s biggest scenes, Coppola tapped Knoxville’s pranking advice. “That’s really his art form,” she told The Post over Zoom. “And so much of the content in this [current internet] world is an homage to ‘Jackass,’ creating these kinds of stunts.”