The 69-year-old actor was photographed leaving the Four Seasons hotel in Atlanta with a script in hand, and later joined the cast on set, a source tells PEOPLE.
One of the major cliffhangers from the 2014 Marvel space romp was the identity of Peter Quill’s (Chris Pratt) father. While fans speculated the role would go to recent cast additionKurt Russell, Stallone’s possible involvement is keeping the mystery alive.
In addition to Russell, Director James Gunn and Marvel also confirmed Pom Klementieff, Elizabeth Debicki and Chris Sullivan will be joining the cast.
If he is indeed on board, this will be Stallone’s first turn as a superhero, despite playing numerous action movie icons over the years.
Guardians of the Galaxy Vol. 2 opens in theaters on May 5, 2017.
Reporting by ELISSA ROSEN