Robert Rodriguez is busy shooting Sin City 2: A Dame to Kill For, which is due in theaters on August 22, 2014.   While it was common knowledge that Joseph Gordon-Levitt had joined the cast that includes Josh Brolin, Eva Green, Jessica Alba, Mickey Rourke, and Bruce Willis (see "Gordon-Levitt in 'Sin City 2'"), this is the first confirmation that pop star Lady Gaga will have a cameo in the second Sin City movie.
Gordon-Levitt revealed Lady Gaga’s modern noir cinema debut in a radio interview with Boston radio station Jam’n 94.5 (via MTV news): "She was in Sin City 2.  I did a part for [it]. Mr. Rodriguez asked me to be in, and Lady Gaga is in a scene of it.  And I gotta say, she did a really good job.  Like, she's a great actress.  And I didn't know that.  You know she came in and I was like, 'OK sure, they're having a huge pop star do a scene in the movie, I understand.'  But then she came in and like, nailed it!  She had her character down, and she was like a total pro and did a great job.  I was really impressed."
Rodriguez was evidently impressed as well since he quickly signed Lady Gaga to appear in another film he directing, Machete Kills, the sequel to his 2010 film Machete starring Danny Trejo.