The closing credits indicate that the characters of Mungojerrie and Rumpleteazer were "dubbed" by other singers. Drew Varley and Jo Gibb sang the entire song themselves but they couldn't get the giggling and "Shh"-ing at the beginning of the song down.
The words "Jellicle cat" came from when T.S. Eliot's niece tried to say "dear little cat" and the words "Pollicle dog" came from when she tried to say "poor little dog."
Cut Song: In the stage version of 'CATS' on Broadway, London, and National Tours, midway through the "Gus: The Theatre Cat" song, there is usually a lavish production number known as "Growltiger's Last Stand/The Ballad of Billy McCaw". It is a memory from Gus's younger performing days. Usually there is also a big set change to a pirate ship.
Elaine Paige, Susan Jane Tanner, and Femi Taylor were all members of the original London cast; Ken Page was a member of the original Broadway cast. Page, Michael Gruber, and Jacob Brent are the only American performers in the video.