Mal is going back to space this summer — though he unfortunately won't be aboard the Serenity. Nathan Fillion will appear in Marvel's Guardians of the Galaxy, director James Gunn revealed this weekend on Twitter. The role is extremely brief, says Gunn, who said on Twitter that the Firefly and Castle star has a "a very small fun cameo he did as a favor to me." Fillion had previously teased that he'd make it into the film's credits, and rumors intensified this week after a report from Latino Review claimed Fillion would have a more substantial role in the film as Nova. Gunn put those rumors to bed once and for all by confirming that Fillion's involvement is limited to a cameo. Fillion previously worked with Gunn in 2006's Slither and Super from 2010.