In some early scenes where Keanu is running, the running cat is obviously older and larger than the kitten seen during the rest of the film.
Clarence (as Shark Tank) tells Cheddar he should keep his safety on. The gun he is using can't have the safety engaged with its currently un-cocked hammer.
When the kitten escapes during the action scenes, you'll notice that his tail is upright. That is the sign of a happy cat. If a cat was truly scared, the tail would be low to the ground, and very fluffed up.