We've all met them. Characters who just lie "blah" on the page. Sometimes this is okay, say for instance, when the character is an "extra" with a very specific role, never to be seen again. Like the paper boy who delivered the newspaper with the headline, "ALIENS FROM JUPITER LAND IN PODUNK, KY!" Obviously it is important the newspaper arrived on the doorstep of the only human to speak Jupiterese, but how the paper got there probably isn't. Thus the flat paper boy. Most readers will imagine a perfectly suitable paper boy and go on their merry way, more interested in the Jupiterites (Jupiteririans?) and the linguistically gifted main character than in the kid who delivered the news.
But, what if the paper boy plays a larger role in the story? Should he remain one-dimensional? Absolutely not! Once he takes on the role of alien-liaising sidekick, Bobby the Paper Boy needs to be fleshed out. Now, that doesn't mean you should just run-down a de facto physical description of the kid. No. Show your readers how his red hair stands on end from sweat when he runs his hands through it while he's thinking. Give him a lucky rock in his pocket, worn smooth from subconscious fiddling with it when he's anxious. Let him decorate his now, too-small bicycle with stickers and playing cards which flash and crackle as he speeds up and down the neighborhood streets in a flurry of knees and elbows. Is he a street-wise smartass or a wide-eyed innocent?
How deep you delve into Bobby depends on what voice you choose as an author. Third-person omniscient will allow you to tell your readers what he (and all the other characters, too) are thinking or feeling. Third-person limited will pull you back a bit and let Bobby do some thinking on his own, with his actions bearing out where his mind is. Choose your point of view and voice wisely and let your characters breathe with their own unique personalities by showing your readers who they really are. The details really do matter.