A term coined by John Green in his novel Paper Towns, and thus embraced by nerdfighters everywhere as a truly magical gesture of the miracle of human consciousness.
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.
As Leo watched people at the bus stop on a foggy London morning, he imagined the pixie-ish girl of about his age complexly, and suddenly felt he understood the wanderlust behind her sullen, deep set eyes.
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.