Major disclaimer here, I am primarily a plot driven writer and I had thought characters were my weak spot. Although in the beta readings of my latest book MIND THIEF people are loving the characters and dialog and missing the plot because of it. So take these words with a grain of salt.
You've figured out what POV you should use and where to start now you've got to have the who, your main character. In the pulp sci-fi novels of the 50s I grew up on that wasn't a problem. Characters were one dimensional. They had to be because of the way they were written. The writer banged them out on typewriters and often didn't do rewrites as rewriting anything back then meant if you wanted to change a word or two you would have to retype the entire chapter.
So if a writer described his main character as having shoulders like a linebacker in the first chapter and then the hero had to roll under a closing blast door that was a foot from the ground, the writer would have to go back and retype the entire first chapter.
The same is true for the main character's motivations. The main characters had to be heroic and pure of heart as the more subtle motivations could trigger a rewrite. The best example I can think of bypassing more subtle motivations is MYSTERIOUS ISLAND. The five men lived on an island for years and only thought of making it a better place to live. A writer today could never get away with that as critics would ask, why aren't they breaking down and having prison sex on the beach?
So today a writer needs to give their characters more motivations than just being Dudley Doowright. Unless you've made your character have a reason, like OCD, to have a laser like focus on the final goal, their motivations need to be more than just doing the right thing.
One thing that gives your characters life is the initial decision, the choice they make that sets everything off. The second thing that makes readers identify with a character is giving them a motivation that the reader can relate to. It doesn't have to be something they experienced but that they can empathize with.
My main character, Howie, in MIND THEIF was adopted and his adopted dad disappeared when Howie was 15. As a result he doesn't trust people so he tries to do everything by himself, including having to get a 3.8 GPA to maintain his scholarships and enrolling in a psych study to pay for the rest of the bill.
Now that is something I've never experienced as my tuition, rooms and meals cost $1,600 per semester and my Pell Grants were $2,100. Back then you only paid for college if you had a specialized major, or were from a wealthy family.
But I can empathize with Howie and see why he would choose to enroll in a psych study.
Howie not wanting to rely on anyone isn't an uncommon motivation. People can see why all his decisions can stem from that motivation.
Naturally this applies to all the major characters in your book. With the secondary characters you can have more fun as you don't have to show their motivations up front. When you finally do show their motivations the reader can look back and say, that's why they acted like that.
When thinking about what motivates your characters keep it simple and something people can relate to. That can lead to the reader cheering for them in the final conflict.
A classic example of using a simple motivation to make the viewer cheer for the heroine is ALIENS. Ripley lost her daughter and she runs into Newt who needs protection from the aliens. Ripley's motivation is clear she isn't going to lose another child. It brought the movie up from just a Humans Vs Aliens movie to two mother's battling to save their children from each other.