I have listened to many hours of the German course but the dynamic between the two students is distracting. They essentially play 'the genius' and 'the grouch'. The good one remembers everything. The bad one snorts and grumbles at every question. The acting is so forced that it completely breaks the immersion.
But I don’t think it is acting. He just only did one take. Usually someone is better then a the other. That’s what I understood from the documentary anyway. He just did the tapes because he was old and someone bugged him enough.
They seem so perfectly weighted and with exactly the right increase in difficulty that I assumed they must have been heavily edited / selected.
But basically he kept his methodology secret - literally locked in a safe - because he didn’t trust anyone after his experience in ww2.
It wasn’t until he was really old that someone convinced him to make the recordings. And the tapes are just that.