Strong characters are vital to a strong story, so don't scrimp on their development.
Developing characters is one of those things where everyone has their own method (which is totally fine). But it also often takes a writer multiple tries to find that perfect process that works for you. No matter what you might read out there, there's no right or wrong way to do it.
Some folks love to interview their characters, others make lists of key traits, some fill out pre-designed forms with a variety of details, and even find photos online of what they look like (I find this photo bit particularly useful).
One friend of mine creates collages that represent that character, letting her mind find images that feel right to her, then thinks about the kind of person who'd create that collage. Another friend dives deep into the emotional states of the characters and needs to understand how they tick before she can do much with them. And me? I only know the bare bones going in.