While his 28-year-old fiancée watched from the audience, the 51-year-old actor grabbed a guitar and helped Aerosmith play "Train Kept a Rollin," their final song of the night, a source tells E! News. According to the eyewitness, fans "went nuts" when Depp popped up on stage to give concert-goers—and Heard—a performance to remember.
The star-studded musical moment was made possible by the fact that Depp is currently filming Black Mass, the upcoming movie about Whitey Bulger, in the area.
This certainly isn't the first time Depp has rocked out on stage with Aerosmith. In August 2012, he joined them during their Hollywood Bowl concert and played the same song with them.
And in Dec. 2012, he performed "Stop Messin' Around" and "Come Together" with them during another show in L.A.
—Reporting by Beth Sobol