Imagining someone complexly is imagining them in the terms you might imagine yourself. Things you think about your person, such as dreams, histories, heartbreaks, epic loves, and deepest secrets, are also things you ponder about people you are imagining complexly.
Had he not imagined her complexly, his thought process could have probably been something a bit more like "She's probably tired. Kind of pretty, though..." and he would have moved on to the next commuter.
Always imagine complexly.