Stories are character-driven. You may start writing with a scenario, but it has to be peopled. And those people need to be interesting, because no one wants to read about dull people. Dull people are everyday life, and they never make the news.
Most of the time the characters are going to be exceptional in some way, although there are good stories about ordinary people thrown into extraordinary circumstances. Alice In Wonderland, for example. Or Arthur Dent in Douglas Adams' Hitchhiker's Guide to the Galaxy. But even those two weren't dullards who sat in the middle of the plot while things happened around them; the ordinary people have to have some qualities worth observing, otherwise the reader has no sympathy for them and will lose interest.
An example of doing it all wrong would be the movie version of Gone With The Wind (1939) where other than Clark Gable's depiction of Rhett Butler all the main characters are dull and unsympathetic (especially the supposed star, Vivian Leigh and her portrayal of Scarlet O'Hara as a frankly horrible person with no redeeming qualities). The secondary characters are all more interesting, and better acted. It's no wonder Margaret Mitchell didn't like it.
An interesting inclusion of 'ordinary' people is in the long-running sci-fi show Dr. Who, wherein the best 'companions' are inevitably human - because that allows a link between the alien adventure and the typical viewer. A human to explain things to and be brought into the action gives the audience a personal bridge into the story.
But how do you write an interesting character? Frankly, it's not easy. It helps if you have known a large number of people from many walks of life, have observed them well, and have studied psychology. No one character is going to be an exact copy of someone you know (unless you like getting sued), but various aspects of many individuals will be drawn into a composite figure. This has to be plausible, of course. Too much dichotomy, too much eccentricity, and you will have a farce rather than a novel. Of course that may be what you are going for.
Another difficult thing with any character is introducing them. Simply slamming them into the narrative with a biographical description lacks both subtlety and finesse:
"Ronald Cornet was an 80-year-old white man who stood 6' 2" tall and weighed 235 lbs."
Yeah, I fell asleep just reading that. You may write a character profile like that (in an even more analytical format) but no one wants to read what sounds like an APB from the police. Not only do you need to be more subtle about it, but you have to remember not every last detail needs to be included. Sometimes it's important to describe the person, sometimes their depiction is relative to other characters, and always the reader is going to apply their own interpretation - so let them.
Also, sometimes the characters apply their own interpretation, both to themselves and others. Angel thinks Amy is tall, and guesses her height at 180 cms. Amy corrects this, being 'only' 173 cms. To Angel, who is only 157 cms, Amy is tall. Beyond that, her admiration means that even if they were the same height Amy would be considered tall by Angel. In contrast, Angel doesn't notice how tall Amy's brother is (193 cms.) because he is not as important to her.
(And by the way if you've just deduced that Angel and Amy are lovers, you got it wrong.)
What's even more important than the physical description is, of course, the personality. Every character has to have their good and bad points in order to be believable. "Bad points", by the way, does not mean "secretly evil" or "kicks puppies when no one is looking". Merely that all of us have aspects of our personalities that aren't our best features, whether it's not being good at some function or a dislike for a certain thing; there are no saints, even among saints. It may be a foible, a weakness, a failing, or a fault but there will always be something. That's what makes us, and our characters, human. Of course if you're not writing about humans this may not apply.
To introduce characters into the story you have to look at them afresh. Never mind that you've already constructed their full biography and backstory: what is the first impression they have on the others and indeed the reader. Some people may be spotted sitting quietly in a corner, others burst into the room and take over the situation. And as the plot progresses, more aspects are revealed as the situation warrants.
Writing a good character is the most difficult part of writing a story, and the most important. Try reading Samuel Beckett's "Waiting for Godot". There are only four (as I recall, could be wrong) characters and practically nothing along the lines of set or props, yet it is considered one of the most significant dramatic works of the 20th century (although there are those who disagree). Why? Because of the characters and their interactions.