If by "round" you mean three-dimensional, which I think is the far more common term, it's not the same thing as dynamic. Just like real people, who are all three-dimensional, but who are not all dynamic, characters are the same.
In writing, "flat" characters are most often called "cardboard".
But the short answer is that the same thing makes a character three-dimensional and/or dynamic that makes real people the same way.
In the poem called An Essay on Man, Alexander Pope has the line, The proper study of Mankind is Man.
In writing, the proper study of Character is man. A three-dimensional character needs the same traits that makes a real person three-dimension. He has good traits and bad, pimples, bunions, bills to pay, grouchiness, temptations, and imperfections' Sometimes generous, sometimes selfish, sometimes reasonable, sometimes unreasonable. He trips over cracks in the sidewalk, he likes cats and hates dogs, or the other way around. He has a real life outside whatever problem the story throws at him. He makes good choices, he makes bad choices, and he farts. He may not fart on the page, but any reader knows he does have to fart occasionally.
Too, "dynamic" is a personality trait, one some people have, and some people lack, one some characters should have, and others should not. All characters should be as three-dimensional as space allows, but all characters should not be dynamic. Often, not even the protagonist should be dynamic.
As someone else said, you need to know nothing about a character before you start writing. I never do. My study of Character is Man. I don't create characters to put on the page. I'm neither God nor Victor Frankenstein. I put people on the page. I know myself well. I know my perfections and imperfection, my wise choices and my stupid choices, etc. And I've known a lot of other people in my life. I've known them well. And whether in articles, newspapers, journals, or autobiographies, I've read abut many, many others.
So I put people on the page.