Sentence:  Their friendship (immortalized in a splendid volume of letters that has clearly served as one of Ms. von Trotta's sources) is a fascinating study in cultural and temperamental contrast, an impulsive and witty American paired with a steady, <WSD>phlegmatic</WSD> German. 
Meaning of the word marked with <WSD> tags:  Not easily excited to action or passion; calm; sluggish.