In Peter Schaffer's Lettice and Lovage, a cat is numbered amongst the cast. In the original production, Maggie Smith played the owner of this feline, and two very handsome marmalade pedigree cats  - brothers named Fortnum and Mason, IIRC - were engaged to play this essential role. I no longer remember which was which, but at the first dress, the cat originally chosen as the performer was discovered to be lacking in theatrical aptitude, and was replaced by the understudy, his brother, who was far more placid in temperament, and created no problems. One day however, something gave him a sudden fright and he leapt from Dame Maggie's arms, shot through the fireplace and could be heard racketing about the backstage area for the rest of the scene until he was eventually coaxed and captured. Thereafter his sibling was entrusted with the role again, and this time behaved purrfectly.

No Fortnum nor Mason, but a very welcome clip

One birthday, long ago, Mort, I had a horrendous audition at BH which went very, very wrong. To cheer myself up, I walked down Regent Street to Shaftesbury Avenue and the Globe, to see if there was any chance of getting a ticket for the evening's performance. There was a long queue outside the theatre, so it didn't look likely. But someone came down the queue asking whether there was anyone who wanted just one ticket. All the rest ahead of me were couples, so I went to the front of the queue, were I was offered the middle seat, front row Dress Circle. The evening more than made up for the rather traumatic late afternoon. Totally outrageous.

http://en.wikipedia.org/wiki/Lettice_and_Lovage