If you taboo the word 'personality', the underlying question here is still interesting. Do different people in the "exact same" situation always behave /identically/? (Obviously not.) What are the most informative features of the person explaining the divergence in response?

Christian Miller is arguing that while we do have phycological character traits and motivations, that persist between situations, how we actually behave is a complex interaction between these and our contexts. Divergence can maybe be explained as much by contexts as by characteristics. If I were to meet someone who works a similar job, has a similar social and familia situation, but divergent motivations, I might have more in common with them than I would have if I met the university student version of myself.