Producer Katsuji Morishita discusses working on Tsubasa Reservoir Chronicle and xxxHolic as two halves of a double feature project. Two particularly interesting tidbits here (among a few others):

-Tsubasa character designs were redone from scratch for the movie.

-Tsubasa movie director Koichi Mashimo was good friends with TV series director Itsuro Kawasaki, which helped smooth the transition between teams.