The following is an excerpt from an interview with Rain right after the performance:
Q. When did you get the offer to perform in the closing ceremony?
A. A year ago, I was asked to perform with an American artist. Later, they changed the theme to "Asian Unity" and decided to invite Chinese singers to perform. The organizing committee asked me to represent Korea. I later heard that film director Zhang Yimou, who was the artistic director for both the opening and closing ceremonies, recommended me. The plan was finalized four months prior to the Olympics, and I met with the singers a month before to record all the songs. We sang two lines each and sang the chorus together.
Q. You've performed in numerous large-scale concerts throughout your career, but isn't this your first time performing in front of 90,000 audience members and billions of TV viewers?
A. I performed in front of 40,000 audience members during my Beijing concert. However, my heart started pounding at the thought of having to sing in front of 90,000 people from all over the world and the sheer size of the whole performance. Each and every one of the singers I performed with on the evening of the closing ceremony was extremely passionate. Athletes became one and enjoyed the whole party. I felt electrified and excited at the same time. It's been a huge honor.
Source:KBS Global
http://english.kbs.co.kr/entertainment/news/1542685_11858.html