Adding Characters

   I was reading a book review the other day about how a writer threw in a new character on page 170 and the reviewer wasn't happy about how the writer added this new character. I haven't read the book so I don't know exactly how the character was introduced into the story line. But what I'd like to know is, "Is there a certain protocol or etiquette a writer should follow when adding new characters?"

The reason I'm asking is because I too have added new characters further into my current novel, but have introduced them in a way that lets the reader know who they are and why they are there. I find that these new characters are important to the story line at this point, but they also might not make another appearance in the remainder of the story.

I look at it as watching a movie, you might have a character be there for 5 minutes and yet they are a vital part of the story line.