Rodriguez worked with the Born This Way hitmaker on last year's (13) MAChete Kills, and her stellar performance prompted him to recruit her for the Sin City sequel.
The director even suggests Gaga should pursue a career in Hollywood, telling E! News, "I told her she had to be in it (Sin City 2)... The camera loves her. She's a great performer. She's performed twice for me now and I was blown away by her discipline. She studied acting before she became a singer and you could really tell."
Gaga, who briefly attended New York University's prestigious Tisch School of the Arts before dropping out to pursue a musical career, was in the middle of her U.S. tour while Rodriguez was filming in Texas, so he found a small cameo role for the pop star.
He explains, "She happened to be on tour in Houston, just a couple of cities away from us in Austin. She was ready. She brought her own wigs! I said, 'We're not going to have time to make a wig for you.' She showed up, we did the scene, she knocked it out of the park and then she was back on tour."
Sin City creator Frank Miller also added his own praise for the singer, saying, "She totally transforms into the part she's playing to the point that it doesn't even feel like acting."
Gaga plays a waitress in Sin City 2, and shares the scene with Joseph Gordon-Levitt. The film hits cinemas on Friday (22Aug14).