The 34-year-old actor – who portrays the titular hero and his alter ego Hal Jordan in the movie – wanted to keep a souvenir from the blockbuster but because the production used so much CGI, he could only take a ring.
He said: "The weird thing about this one is that I completed the whole film and then 75 per cent of the film-making process began when I'd gone home. To me it was just four green walls, yet to everyone else, I'm on a 3D alien planet.
"There was nothing actually there. It's all added in post-production.
Ryan admits he is feeling a huge pressure to live up to expectations for the film – including his own.
He added: "I'm a little bit nervous. There's a lot of expectation to deal with on a movie this big.
"Even my own expectations are quite hard to deal with. But for the most part, the hard work is done, so what can I do?"