People close to both men, including Mr. Thaler and Ms. Tversky, say Mr. Lewis captured the intensity of their relationship and their individual quirks. Colleagues described how the pair would finish each other’s sentences and could often be heard cackling from behind an office door as they wrote dense academic papers. Mr. Tversky was the bold one who delighted in undermining well-established dogma within psychology. Mr. Kahneman was cautious, sensitive and deeply pessimistic.