For context, I'm writing an essay in English on a Japanese movie about a particular family that goes by the name of Sasaki. In the movie, the characters are mostly referred to by their family name. The husband in particular is exclusively referred to as Sasaki, though he does in fact have a given name, Ryûhei.
Firstly, my question is, how do you generally refer to Japanese people when writing in English? Do you use just their family name, just their given name, or both?
Second, if you use just their family name, is it appropriate to use both names to distinguish between members of the same family? If so, is it okay to shorten their distinguishable names to just their given name after referring to them by their full name at least once? For example, the first time I refer to the husband I might say "Ryûhei Sasaki", but each time afterwards is it okay for me to simply refer to him as "Ryûhei"?