'50/50': How Joseph Gordon-Levitt Signed on Cancer Comedy at Eleventh Hour (Video)

Joseph Gordon-Levitt hits theaters Friday in 50/50, a cancer comedy written by Will Reiser, who drew on his personal battle with a rare form of that disease.

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But the actor revealed in a recent roundtable chat with Reiser and co-stars Seth Rogen, Bryce Dallas Howard and Anna Kendrick that he wasn't actually part of the cast until the movie was already in production.

VIDEO: Watch Seth Rogen and Joseph Gordon-Levitt in '50/50' Trailer

"It all happened kind of normally except at an accelerated pace," he said. "I got a script but I had to read it right away and decide right away because you guys were already like four days into production."

Quipped Reiser: "I think we gave you 90 minutes to read the script."

Added Gordon-Levitt: "But in the end, it was this really fantastic opportunity to do a good movie because I ended up really liking the script and flew up to Vancouver that night to go meet you guys."

FILM REVIEW: '50/50'

Rogen was friends with Reiser before he was diagnosed six years ago and even serves as one of the movie's producers. But Reiser actually gave credit to Howard for coming up with the film's title.

"That was the most stressful aspect of making the movie," Reiser said.

"It's about half some things and half other things," added Rogen by way of explanation.

50/50, from Summit Entertainment, also stars Anjelica Huston, Serge Houde, Andrew Airlie, Matt Frewer and Philip Baker Hall.