Thanks to word.a.day I now know what (or should I say who) a best boy is.

From: Wordsmith 
To: linguaphile@wordsmith.org
Subject: A.Word.A.Day--best boy
best boy (best boi) noun

  The first assistant to the gaffer (head electrician) of a film crew.

[Apparently borrowed from the sailing terminology.]

 "Cast and director commentaries are one thing, but do we really need one
  from the post-production team, as we've seen on the extended edition of
  The Lord of the Rings: Return of the King? What's next? 'Reflections
  from the Key Grip and Best Boy?'"
  Mike Snider and Thomas K. Arnold; Press 'Repeat' or 'Skip'; USA Today
  (Washington DC); Dec 28, 2004.