Characters have two jobs. One, they must respond as real people so we can identify with them. Two, they must function as part of your plot to they contribute to the message.
Characters who don’t ring true drop your readers (or audience) out of their involvement with your story.
Characters who don’t have a plot function seem pointless and can disrupt the flow of your story.
That being said, there is no need to develop the personality of a character who is simply a vehicle of exposition to provide some necessary information to your readers.
Similarly, characters can provide color and passion to a story, even if they have no impact on the course of events.
Think of these two approaches to character as the “play by play” and “color commentary” on a sporting event. One announcer tells you what’s happening and how it fits into the big picture. The other announcer provides interesting information about the backstory and personality of each player, helping us see them as people, and drawing our interest and involvement.
In your story storytelling, review your work from time to time to ensure your critical characters are working to advance the plot. And then take an emotional picture each character in your story verify that they have sufficient personality traits and personal information to attract your readers, hold their attention throughout the story, and lead them to identify with your characters or at the very least, identify them as a “type” they see in everyday life.
More on character types in future Beginning Writer Tips from StoryWeaver.