Ok, So I know you have all heard the "Don't make your POV characters two dimensional. They have to have flaws in order to make them relatable." speech. And I'm sure that every new writer out there has gotten the clue. You don't make your characters two dimensional. You just don't. 

But what about your side characters? Maybe they're your character's best friend, maybe they're a classmate or teacher, maybe even a ruler or government official that is standing in your character's way. Whoever they are, we rarely ever pay much attention to them. When I say 'we' I mean writers. Readers however...Readers will pay attention to everyone and everything. 

Your side characters are the protagonists of their own story. They have lives, emotions, good and bad days. I know its so easy to fall into the 'best friends are always there for each other' trope, but in order to make them relatable to you character and to the readers we have to give them the same time of day that your main character gets. They are not just there to validate your POV character's feelings. 

Example 1.

Lacy flopped onto her bed with an exacerbated sigh. "If he hates me, and I hate him then why do I feel this way?"

Lillian groaned, "Not this again. Lace this is the third time this week."

Lacy rolled over with a groan of her own, "Why is he all I think about?" 

"I don't know, maybe you actually like him and you're afraid to admit it?" Lillian cocked an eyebrow as she sat on the beanbag in the corner. 

"Maybe you're right Lil" Lacy muttered, covering her red checks with her palms.

"Dude, I'm always right." Lillian chuckled as she tossed a pillow at her friend.

Now this was a scene that could have given us so much more information, not just about their friendship, but about Lillian. Lets try another one.

Example 2.

 Lacy flopped onto her bed with an exacerbated sigh. "Why does everyone think we're dating? He hates me and I hate him!"

"No you don't. All you ever talk about is Jason this, Jason that. Honestly if you actually hated him it would make my life so much easier..." Lillian paused and looked down at her nails thoughtfully,  "Actually, no it wouldn't. I ship you guys too hard." She chuckled. When Lacy didn't respond Lillian heaved a sigh. 

"Just admit it Lace, you've always liked him. You just never have the courage to admit it." 

Lacy growled, rolling over onto her stomach to glare at her friend. "Says the one who waited five freaking years to tell her crush that she liked him."

"Hey! To be fair it was only four and a HALF years not five. And I was waiting for him to make the first move only he was too slow." Lillian snapped, returning the glare with the utmost ferocity. 

Lacy groaned, burying her face in her pillow. "I hate this!" she yelled, "I hate this feeling! I hate his stupid beautiful face!" 

"No you don't" Lillian replied with a smirk, eyeing her chipped red nail polish with distaste. "You only like to think you do."  

Do you see the difference? In example 1 the only thing we hear about is Lacy, Lacy this, Lacy that. We get no insight into Lillian's character, or what she means to the POV character. But in example 2 we get to hear a bit about Lillian's past, and the slightly ironic nature of what she's telling Lacy. Which Lillian do you feel more connected to? The one we know nothing about? Or the one we get a glimpse of? 

Side characters are extremely important to the plot, wether they seem to be or not. Maybe this story is all about how Jason and Lacy fall for each other, and Lillian's just there so that the readers get a glimpse of whats going on inside Lacy's head. But we oftentimes forget why Lillian was there in the first place. She and Lacy became friends somehow, and by the sound of it they know each other pretty well. 

How did Lacy and Lillian become friends? Why did Lillian wait four and a half years to ask her crush out? How does that affect her advice to Lacy?

While you don't need to spend a huge amount of time on these issues, it is important that the readers get a sense that Lillian is a real person, not just a plot device. So before you add your secondary character into a scene, figure out what connection they have to those events. 

Lillian is the protagonist of her own story, which means that there is a reason behind everything she does, and some of her habits have probably rubbed off on Lacy. Maybe Lacy chews the ends of her pencils because Lillian used to do it, or maybe Lillian taught her how to paint her nails. 

It also means that Lillian has a life of her own. Maybe she has a part time job at a hair salon, or maybe she's trying to save up for her dream car. Maybe she got into a fight with her boyfriend recently and needs to rant about it. Just like real relationships, fictional relationships are a two way street. Lillian isn't just there to listen to Lacy's problems, only to be absent for the majority of the story and Lacy can't just call Lillian up whenever she feels like it. 

Look at it this way, if you were to call your best friend right now would they answer? Maybe some would, maybe some wouldn't. Your friends have lives of their own and can't always be at your beck and call. The same goes for secondary characters. They are their own person. 

So while the whole writing world is screaming "Don't make your POV characters two dimensional!" I want to advise you that the best way to make your POV character three dimensional, is to surround them with three dimensional side characters.