Note!

Rating: 10/10 (18 August 2001)

The only basis on calling this character Jack is when he is mimicking those point of view pieces on anatomy, etc.
NEVER is the main character actually called Jack, in fact, he never uses real names. (as far as i can tell)
I believe this was intended as a stylistic element, as the credits list him only as Narrator
in the book, it is done this way as well (the author writes screenplays as well, and wrote the screenplay for this movie as well, of course)