(written from a Production point of view)
According to Robert Lansing, "We had three black cats. That was because in those days, the theory was that you couldn't train cats. Cats would have a certain propensity: One would like somebody, would want to follow them around, so that day, you would release the cat that would probably do what you wanted it to do. One of the cats took a great liking to me. It was always loose on the set when I was working, so it happened that the stuff on the rocket gantry was all ad lib. I would say something like, 'Isis, come on, you're getting in the way. You know, there is a bit of a hurry. This is not the time to be jealous.' We added meows in later."