Having written several ongoing series’ (Maudelayne is entering its fifth season for example) there inevitably comes up how to introduce a new character. This can also come up when writing a series of books or short stories. Sometimes a new character is there to fulfill a purpose—love interests for example—so how can they be integrated without being a cardboard cutout that is obviously “oh look Jane Doe has a boyfriend now”. When I added Krinaia to the Maudelayne cast, I gave her a few episodes right away that showcased her unique talents that were needed by the ensemble. I wanted to make sure the listeners could connect with her as a strong individual character apart from the fact she ends up marrying Atherton. Doing another random hitjob on my Netflix queue I looked up how some tv series’ added in new characters to their ensemble casts.
Buffy the Vampire Slayer—Tara
Sometimes a minor character turns out so fascinating that they slowly build and one day you blink and there they are a regular cast member. That’s what happened with Tara—she is introduced as an acquaintance of Willow who also has an interest in magic, is thrown into the episode Hush to help with tension and plot building, and slowly shows up more and more with Willow as the plot line of magic is needed. Originally she was a character of convenience, but her incredible chemistry with Willow and just how “right” the two of them looked together ended up slowly building into the two of them beginning a serious relationship. By the next season, Tara was just naturally a member of the cast, and that was highlighted when she finally got an episode with a storyline all about who Tara was, and where the main ensemble demonstrates how much they care about her. It is one of Joss Whedon’s famous moments as a group of disparate characters come together as a tight-nit family. Adding a character isn’t always about planning—sometimes a minor character blossoms into a main cast member and it is up to the writers to recognize that and give the character room to grow.
Legend of the Seeker—Cara

In season two, a major new character joins the ensemble, but she was hinted at beginning with the climax of season one. They took the episode a lot of people would be paying attention to—the season finale—and threw her into the mix. The writer did not waste any time and introduces Cara in the first scene. Her first lines are to contradict the most feared character in the series who actually accepts her advice, demonstrating she is fearless and intelligent. The biggest question would be why such a strong character was not in most of the season, which is answered in the same scene as Cara demands to know why she was not summoned earlier. She moves straight into the acting without hesitation and she becomes the primary person driving the plot of the finale. When she arrives as a regular character in season two, it is difficult to question her since she basically burst in fully formed as a formidable character.
Stargate SG-1—Vala & Mitchell
With several main cast members leaving the show, SG-1 faced the challenge of trying to recast them or bring on entirely new characters. They chose the latter (which I agree with after the fiasco of recasting Teal’c's wife several times).
Mitchell was a totally new character pulled out of nowhere and set to lead the team. Replacing the main hero on a show is extremely difficult, and rather than bringing someone up out of the ranks, Stargate inserted a new creation. He had some things in common—air force officer inclined to be snarky—but he was younger and more naive than the character who proceeded him as team leader. They used the empathy tactic to introduce him—he was apparently wounded when trying to protect SG-1 in a previous episode. They used insert shots into old footage to add to the storyline along with using the character he was replacing giving him the thumbs up for commanding SG-1. Furthering a viewer’s empathy, Mitchell was thrown into the episode as trying to put the team back together and failing miserably. Viewers loved SG-1 and wanted to see the team again just as much as Mitchell did, so it was a good tactic to get viewers to like and accept him.
Vala was a minor episodic character from the previous season. She was a lot of fun and had some great chemistry with the cast, which is probably why they chose to bring her in as a regular. In the episode she was used as the reason SG-1 ends up getting back together. Her comedic timing is excellent, and her flirty light style fit well with Stargate’s writing that is a combination of serious and cheeky. Using a semi-familiar character as the means for introducing a totally new character was an interesting choice that put viewers a bit at ease that they were not going to get force fed a totally new cast. Her specific character traits (or flaws depending on how you look at it) come in handy during the first few episodes, although they also get her into trouble. She added a new dynamic to the show and let viewers have a good laugh which helped with the bit of nervousness that Mitchell’s character introduction created, putting people at ease with the show.
Charmed—Chris

I know what you’re thinking…why am I not commenting on Paige? I wanted to look at different types of new characters added, and just covered replacing a character via Stargate so I wanted to look at a different situation. Chris presents a great example of a character added in order to spice up a show. On a superficial level, Charmed needed some young eye candy and Chris provides that perfectly, on another level he brought in a whole season-long story arc. Another unique aspect of the character was how incredibly mysterious he was for such a long time. You never knew what side he was on or what his ultimate motives were and it made his character’s story that much more compelling. When the truth is finally revealed, I think everyone watching did a massive squeal or at least blinked in shock several times. When rewatching the season knowing that key bit of info (not to be revealed here) all the odd things Chris did suddenly make sense. Basically it was great storytelling. Like Cara in Legend of the Seeker, Chris was introduced during the season finale of the season right before the one he became a regular in. That seems to be a popular way of introducing new characters on a show.
As for the moment in which he is introduced, several of the characters are in grave danger—one of them has literally just been turned to stone, and viewers are on edge hoping nothing worse happens. Right in the nick of time a handsome young man appears out of thin air and saves the day, followed by a quite humorous scene in which he acts like an insufferable know-it-all but also obviously caring about what has happened. He is kept somewhat mysterious right from the first entrance and stays that way through most of the season, but with enough empathetic moments that viewers cannot hate him.
Notes that I learned from looking at character introductions:
— Use the season finale to introduce incoming characters for the next season
— Chose to either gradually introduce someone, or throw them right into the thick of the action, not in between
— It is okay to use an existing minor character and bring them into the main ensemble
— Be sure to cause strong empathy with the new person when replacing a well loved existing character
— Let the new character drive the plot in some way, not be standing on the sidelines

She starts as a powerful mortal queen who was deified and married a Titan, but the others murdered her husband and drowned her son. After discovering this, her daughter no longer wished to live and killed herself. Basileia is said to have wandered wailing through the land until vanishing into a thunderstorm never to be seen again while her two children transformed into the sun and moon.
Maudelayne Series 4