Trivia: The director chose Bruce Willis for the part for his ability to bring certain emotional themes into the diehard movie. He wanted that, but he also made a list of "Bruce Willis Cliches" he didn't want in the movie, including the "Iron Blue stare"
Trivia: During the phone call between Kathrine and Professor, we can see the roof of laboratory. It is made of ordinary aluminium or plastic blocks, like roofs in offices. This kind of roof can transport air. Persons behind glass are doing something with danger microbes and they are in protective suits. Professor and his assistants, before glass, don't wear that suit, but the holes in roof can transfer microbes to their part of the laboratory. (01:27:35)
Trivia: Terry Gilliam was almost killed in a horse-riding accident during production of this movie.
Answer: The tattoo is for the movie. It is his prisoner identification.