By adding in other characters who emphasize the themes of your story, you can add depth. This serves a double purpose by also adding tension. If you're writing a romance, for example, you have the couple together then apart, then bring them together perhaps a couple of times before the happily ever after ending. If you add in a subplot with separate characters who also have a relationship you highlight the romance angle and delay the reader's satisfaction of finding out what happens next. The tease is most always going to keep your reader turning the page as long as it eventually pays off.
Sometimes during the early drafts a writer might not be able to see where all the clues and tension can be instilled for greatest effect, but can address this during revisions.
A good editor will be able to assess your story to give you feedback on where you can improve this angle of your writing.
Questions? Comments? Leave it here or email me at email@example.com.
Writing is a step by step process. Whatever step you're on, enjoy, learn and celebrate!