When I create a character I do the usual type of analysis - what do they look like? what do they want? what is their greatest fear? - you get the idea. But recently I've been paying more attention to how the other characters perceive them. And how my character thinks other characters perceive them. You can see where I'm going with this.
When writing in subjective third person, and especially first person, we must be very aware of how our characters perceive each other. If the main character thinks someone is trying to steal their boyfriend/job/sandwich then they are going to see them in a completely different light than if they think that same someone is simply interested in their life. When we see a world through one character's eyes, we can use this to our advantage. Presumably we want the audience to empathise with our main character and so if they see someone in a bad light, so is our reader. This can be a lot of fun to play with, especially considering all the relationships between different characters and how we can portray them to the reader which may or may not be accurate which can then be used to our advantage later in the story.
So consider how each character sees all the other characters in a story and you can make your world so much more believable and fully-formed, which equals a better told story.