There's no iron-clad rule saying what comes first when you are writing. There are people who outline an entire book - they may also created complex histories for their characters and descriptions of every scene.
Personally, I start with a situation. I throw my characters into it and see how they develop and deal with it. In Tough Times, I wanted a teenage boy to deal with taking on responsibility for his young siblings, with racial issues on top. In Running Away: Maggie's Story, the situation I wanted to show was one I encountered when I was a probation officer for teenage girls - several of them had been abused by someone the mother brought into the home and, because they'd been acting out, their mothers didn't believe them. In both those stories, the characters kicked in and dealt with the situations in ways I'd kind of expected.
That doesn't always happen. When I started writing Peg's Story: Detours, I knew she was Maggie's mom, she'd run away at the same age, and her parents thought she was dead for ten years. I wasn't sure why she ran away or why it took her so long to go home, but I thought she'd pretty much follow my detours through my twenties. Well, as I met her parents and her best friend (secondary characters), the reasons for running away fell into place, but when she got to the bus station, Peg took over and became her own character with her own story. I still got to use a few bits from my own journey, but her story is definitely not what I expected.
In On Writing: A Memoir of the Craft, Stephen King says "The situation comes first." (p. 164) and that he lets the characters take the lead as they develop.
Nice to know it's not just me.