In one of my current works in progress, a ski novel called The Fall Line, the main character, Mia, let me know that she and the hero, Creech, already knew one another. And that they had a kind of short, unrequited romance in the past. It wasn't something I knew about before I started writing and learning it has informed the story.
Here's Creech's take on what happened between them ten years ago--
I sprinted down the street, turned the corner to the hotel, and sure enough, there she was, walking along. “Mia, wait.” I barely got the words out; the hard run had taken my breath.
She heard me, though, she turned and waited. I had no real plan about what I’d do once I caught up to her; I slowed to a walk so I could get my breath back. Her hair tossed around her long and loose and her cheeks were red with cold. “What’s wrong?” she asked.
“Nothing,” I said. And I put my hand to the back of her head and kissed her.
It was an impulsive thing to do, but it was the right thing and I could have stood in the cold all night long touching the silk of Mia’s hair, her warm lips on mine, her fingers causing pinpricks down my spine as she caressed the back of my neck.