The comedian and Oscar winner joined Billy Joel on stage for his concert at Chicago's Wrigley Field, where the actresses climbed onto Billy's piano and danced to a performance of his hit song "Uptown Girl" (a song that also plays a big role in Amy's film Trainwreck).
Now that really is a sign of true friendship -- or maybe Jen is just an adorable weirdo.
Amy and Jen actually spent the whole day getting ready for the big night. The two rehearsed with Billy in an empty Wrigley Field hours before the concert.
It seems the two celebrated stars have been spending a lot of time together in recent weeks. Aside from Jen joining Amy and her friends on a fun vacation getaway last month -- which included jet skis and human pyramids -- the dynamic duo are also reportedly working on a movie.
On Wednesday, the 25-year-old Hunger Games star revealed that she and Amy, 34, have written a screenplay together and plan to co-star as sisters! Check out the video below to hear more about the potentially epic project.