One of the vital elements for me to develop characters and develop conflict in a story are the relationships between the characters. Romance is all about the relationships between people--and character relationships seem to be the common thread between the stuff I like to read.
Usually, an author has more than one character to play with. There are protagonists, antagonists, secondary characters, and walk-on characters. As they interact throughout the story in various ways, their interactions form scenes, sequences, chapters, conflict and resolution--particularly in romance, which is all about how the main characters interact, process each other, come together and fall away, then finally come together for that Happily Ever After.
I'm hoping I can write a romantic relationship effectively, but I know that reading a lot of romance since my teens means that I can identify a healthy relationship from a not-so-good one. Whether romantic or platonic, soul mates or the way deeper bond of best friends, a healthy relationship consists of back-and-forth communication, giving, honesty, support, understanding, and some shared interests and a shared sense of humor. Attraction helps, too.
Some of the relationships I've written in past stories are: a father and his daughters (as seen in Pearl, through the nanny's eyes) or best friends turned to lovers turned to co-parents (in my one paranormal, which will remain unpublished), best friends or sisters (in more stories than I can name or finish) or a boyfriend and girlfriend with friction between them ("Haunted Lake"). "When Mary Left" was a short story about a variety of relationships, none of them what we would term "healthy"-- there's the thread between Mary and her ex-lover, Mary and her resentment of the child she's carrying, Mary and her feelings of obligation to the people she's staying with, Mary and her apparent rejection by her family.
People act differently with people depending on the relationship. There are different dynamics at play--maybe people are friends because of proximity or through one particular interest or they're enemies because of a very twisted event. Maybe they're obligated to each other or one's crushing on the other. Maybe one has power over the other character. Maybe one character is manipulative and the other can't break away; maybe that's their conflict. Or maybe breaking away from the manipulation is part of the story's plot.
You have to keep backstory in mind, to some degree--how was one person raised as opposed to the other? What is their relationship? How did they meet? Are they close or not so much or do they hate each other? Why are they interacting with each other? How often have they met? Is one a social butterfly and the other not? What happens when their personalities collide?
In my current outline, one character is a warm, happy, confident person--because she was brought up by loving parents and she's naturally even-keeled while her counterpart is moodier, with a much bigger ego, and is awkward around people--because he was sent to boarding school at an early age and his parents divorced and he's not close to either of his parents because he's a movie star and he's been away from home a lot. Their relationship is sort of opposites attract, I guess, but not entirely.