Cillian Murphy didn't think that 28 Days Later was a zombie film when he first auditioned for the part. He was unfamiliar with George A. Romero's Night of the Living Dead, and he's glad he didn't watch them.
In a conversation at SAG-AFTRA Foundation's Conversations program (thanks, Variety), Murphy spoke about this early hit in his career. "I wasn't too aware we were making a zombie movie, to be honest with you," he said. "It was right around the time SARS happened and there was all this 'air rage' stuff going on. So I never felt it was a zombie film. And I'm glad I didn't watch the Romero movies because I didn't realize how hallowed those movies were."
Murphy also talks about the ending to 28 Days Later, saying that they originally shot two versions, one where Jim survives and one where he dies. Luckily, the final cut went with the former option so that we could potentially see Murphy return in 28 Years Later. "I'm available" was all he said to a sequel, but this is enough to give us some hope.