"(They) kept us up all night and snapped us with some wet towels," Stallone said while laughing at the recent Guardians press event in Los Angeles. "They had, us, you know, donate blood and things like that. Actually, it was pretty painless."
Turning slightly serious, Stallone talked about how he is enjoying being part of this new world.
"It was interesting, because I love action films - but I haven't ventured into this genre. So it was quite an experience," he said. "When I walked on the set, you know, you see robots and things and a woman who is 7-feet tall - and I thought it was all great! It was like being on a vacation. It was better than being up a tree in Burma (for his last Rambo film in 2008), you know what I mean? Much better! trust me."
Despite the differences from his other films, Stallone said that, in a way, the Guardians sequel and the Marvel Universe are similar to the type of film he has always enjoyed making.
"Early on in my career, I just became fascinated with mythology," he said. "So, when I started doing Rambo, or whatever, there was an evolution that took and continues to take place. Each generation has to define itself and find its own heroes and its own mythology - and (these films) are this generation's - and maybe even the next generation's - mythology."
"So when (Marvel head honcho) Kevin Feige invited me on board, I said to myself, 'This is interesting, because I haven't really gone here, in space," Stallone emphasized. "I'm always earthbound. I'm terrestial. You know what I mean?
"This is something that takes place in a whole other sphere where James (Gunn, the film's director) and the Marvel people have created their own world, their own reality," Stallone added. "So, I said, 'Yeah, let me visit.Let me drop in here and see what's up (and) see where the future is going, you know?"
"It also got me out of the house from my three daughters, which is why I gave my salary back," he joked.
As for what looks like a bigger role in the franchise going forward, if an after-credits scene is any indication, Stallone said he would be up for it - but will wait to see what becomes reality.