@jim-got-JAMS errr i'm not a very good author but if it is a rom fic, I think trying to make your characters have flaws is the best. The main male lead should be near perfect but give him one very definitive flaw (he can also have minor ones but one that is definitive but not completely evil). Common flaws are: pride, lack of compassion, greed, possessiveness, prejudice,etc. Make your female character be able to somehow help the MC "better" from his flaws, this creates growth in a relationship.
Also overall, know your characters. Worst mistake you can make is making you character's personalities along the way. Give them a background and work with it, make subtle hints to show why they act like the way they act. It builds character growth along the way too :D
Hope that helps, boo!