Ok so whats the joke?

[lifeincoldplaycolor]

i really missed the punch line so i don't really get the humor and why is the crowd laughing....did they think Will is a crew member or smth?
Coldplay Us Against The World Live at David Letterman - YouTube

[zzz]

Chris took the piano stool, so Will didn't have anything to sit on while playing the piano or singing, so he at to sit on his knees.

[jc90]

I thought it was that Chris had to lean back to share the mic with Will

[Cleggy]

Will had nothing to sit on, so Chris and Will laughed and had to recompose themselves. It's like what happened at Glastonbury when Will sang the wrong lyric and they started laughing. I love it when stuff like that happens, it's always a touching moment, especially with an intimate song like Us Against The World.