Developing characters for stories has both advantages and disadvantages. You don’t need to fully develop every character you write about, but if you do you will want to make sure they have their own personalities. It’s fine for several characters to have multiple things in common, but being exactly alike is not recommended.
If characters all had the same personalities, a story couldn’t last. Many stories evolve from personality conflicts and differences of opinion in characters. If they were all the same than if conflict arose it would be as if they were fighting with themselves, and if this were true what purpose would the other characters serve?
While it is possible to create stories based around characters that are all the same, unless you have the very rare skill to make a story like this work, chances are your story won’t keep readers interested.
So, what if you don’t want to define a characters personality? Well, you don’t have to. Really developing a character is about what you want the readers to know about them. Main characters should be fully developed or develop over the course of your story or series so the reader can get to know and identify with them. Often when readers don’t identify with the characters in a book they can’t get into it.
Many readers like to read about fully developed characters, including side ones, but there are reasons why you may not want to do this. One reason may be that they are only needed as fillers. Maybe this character is just a stray dog that a character passes, or a mailman making a delivery, or even one of many servants to a king. Characters that are used as fillers usually serve their purpose then just fade out of the story. The reader then easily forgets about them and feel no need to ask questions about them.
Another reason not to develop a character may be just to keep them from overshadowing other characters, after all you want the main characters to be the most noticeable don’t you? By developing side characters it can take attention away from the main ones. Have you ever noticed that sometimes side characters are just as popular or even more than the main ones? If this happened to you, you would need to try keeping your readers happy who fall in love with and want to know more about your side characters by giving them more adventures with this character. The more characters you develop the more writing you will have to do about them.