Characters are there to serve your story.  They should always serve a purpose.  Sometimes they serve dual purpose.  The hero could simultaneously be the villain.  The sidekick could be the boss, the best friend and the foil.  It all depends on the story you’re telling, but bottom line is, less is more.

It’s really common for writers to introduce new characters when faced with plot problems and sometimes, it’s the right decision.  How do you tell the difference?  Here are a few scenarios you can use to determine if your character is a plot patcher. 

1. The character adds humour to an otherwise heavy drama or serves as a love interest, which is needed, but when you come to write story lines for her, she simply fades in the background.
2. You need a scene to get out some backstory or exposition and create a character from the hero’s past, dump information on him, but never intend to use him again.
3. You need to build body count for a horror film, but there’s not enough time to develop these characters so that the audience cares about them when they die.
4. You have scenes where you introduce a lot of characters who all talk the same and melt into one another.
5. You have characters who serve the same purpose.  You have three detectives when you really only need one.  You have a love interest and a jealous exgirlfriend when this is only subplot. 
6. Seemingly different characters – a villain and a sidekick for example – both of which seem to have independent purpose, but if combined, make for a stronger, simpler story.
7. Journeys or road trip stories by nature have to introduce new characters, but axe them if their only purpose is to point the hero in the right direction.
8. Extras have long speeches or too much dialogue for no reason.

The simplest way to test if your character is a waste of space:

Kill him. 

If your story doesn’t change, he needs to go.

There’s nothing wrong with introducing new characters or with putting a few words into the mouths of ‘extras’ as long as it moves the story along and these lines cannot be given to any of the main characters.

You can introduce characters in episode one with the intention of bringing them back in episode ten, but that intention should be present in the pilot script (or whenever you introduce him).  In specs for existing series, you can introduce new characters provided they leave by the end of the episode.

You can also use your extras to create more important characters in later episodes.  Joss Whedon did this effectively in Buffy The Vampire Slayer.  He cast Jonathan as a student serving Cordelia (an extra with purpose).  Then he later brought him back as the kid in the tower with the gun and even later as part of the trio of evil masterminds trying to kill Buffy.

Audiences know that in film and TV nothing happens by coincidence.  Everything that appears in your story is there for a specific reason.  So if you introduce a new character and he appears to have some history or relationship to any of your main characters, they will naturally wonder, “Who is this guy and what’s his deal?”.   You cannot set up something without paying it off by the end of the story. 

The most important thing to remember is that your lead characters should have the most and the best lines.  It’s why they get paid the big bucks.  You want to put your money where the big mouths are – the stars.