“Thou” is familiar, “you” is formal. It’s a distinction English used to have, but no longer does.

When you hear a character use “thou”, what is their relationship between them and the person they’re speaking to?

There’s a lot of research involved in a good biography.

Interviews with the subject or with other people, cross-checking with historical records, sometimes access to things like diaries or letters…

Here’s the really sad part.

Werth (who was Jewish) survived the war. Saint-Exupery did not.