At his request, David Bowie did not play the piano himself in the "Chubby Little Loser" scene. Pianist Clifford Slapper was playing a second piano off-screen, while Bowie simply mimed the exact movements.
David Bowie and Ricky Gervais were often fooling around off-camera. At one point, after Bowie left the room, Gervais suggested to Clifford Slapper if they could play Bowie's song "Lady Stardust" on the piano. When Bowie returned, he was astonished and delighted to hear the song. As a revenge, Bowie later went to Slapper's piano, saying "Watch this!", and played the intro of his song "Oh! You Pretty Things".
After Ricky Gervais sent David Bowie his lyrics for what would later become the "Chubby Little Loser" song, he called him and asked if he could do something retro, like 'Life on Mars?'. Bowie responded "Yeah, I'll just knock off a quick fucking 'Life on Mars?' for you."