Up to 70% of Nine Lives was computer-generated.
Both Mark Consuelos and director Barry Sonnenfeld are allergic to cats, which made filming scenes with Mr. Fuzzypants difficult. Most of these scenes had to be filmed by the second unit. In the scene where Consuelos is struggling with the cat, his reactions of disgust are real.
There originally was a subplot involving catnip as an allegorical analogue for addiction, but it was cut for time and because executives believed it would be too much for a PG-rated children's movie.
Second movie where Christopher Walken plays a mysterious man trying to save a workaholic from losing his family. The first one was Click (2006)
The breed of cat used for this film is a "Ragdoll" - a breed recently (1963) created in USA. Called as such because you can pick them up any old how and they'll just lie in your arms where you put or hold them. They were (cross) bred to be people friendly (they are) they don't lose their hair (not much) and they don't cry often, or loudly (unlike in the film). His name "fuzzypants" must refer to his very furry back legs that make him look as though he's wearing pantaloons.