The gorilla wasn't in the script. When make-up artist Steve Neill arrived on the set with a gorilla suit in the trunk of his car, Jim Wynorski asked Neill to put on the suit so he could portray a gorilla in the movie.
In an interview with the director, Jim Wynorski, he said "I got my first break doing The Lost Empire for Plitt Theatres. The late owner, Henry Plitt (a decorated war hero), wanted to make a low budget sci-fi action picture as a tax loss. I never knew that when I made the show, so I put my heart and soul into the project. When it finally got completed, Plitt actually liked it enough to give it a wide theatrical release - where it actually made some money."
Angus Scrimm's character, "Lee Chuck" , was so named in homage to Bruce Lee and Chuck Norris.
The reason the two extras playing police officers don't say any dialogue when they inform Karen that her brother has died is because Jim Wynorski couldn't afford to pay them extra money to say any dialogue.