This was before digital so the interactions with "Charon" were not from a computer, but an early BBC character generator called "ANCHOR" which was largely analogue and about the size of a fridge. It recorded the text on paper tape and couldn't animate. However, having discovered the punch codes for stop and start, it was possible to make crude animations by blocking certain punch holes with black tape to change the instructions. This was the first time on-set screens had been fed this way and led to the machine being used on Dr. Who to produce much more complex pseudo computer images.