They cast multiple cats to play Bob but in the end, Bob played himself for most of the film.
One of the reasons Luke Treadaway was cast was because he can play guitar and sing a bit himself. That is his actual voice during the busking scenes.
In one scene Luke Treadaway (James Bowen) is seen feeding Bob Dairy Lea Dunkers. This was Bob the Cat's food of choice during production.
Luke Treadaway (James Bowen) and Ruta Gedmintas (Betty) had been dating several years before appearing together in this project.
Second time James closes the "mouse neighbor door" he uses the book "Of Mice and Men" from John Steinbeck.