At the start, Bill is animated and mostly there. Lines and shapes drift behind him to make an abstract background. The lines and shapes interpolate into images as if we were wandering through his thoughts. Slowly they start to take shape of the room he is sitting in. All the while, Bill is slowly drifting away to become part of those wandering thoughts. The lines and shapes that make up Bill can be the ones drifting away to make the room.  By the end of the video, Bill is gone but the room has taken shape. 
Character Design
Background (wandering thoughts)
Moving Interpolation background style (wandering thoughts)
Reference Ideas