"Sergei Goralick doesn't much like strangers banging on his door. Especially when those strangers are asking him questions." This shows that Sergei is extremely distrustful of outsiders, and even a little fearful of interactions because of his bad experiences with the KGB.
"Sergei doesn’t like this, doesn’t like that the boy is almost atit, already reaching for the jar. In this instant Sergei understands the boy didn’t come for television, what he came for, specifically, is to snatch Sergei’s fish, to steal it away" Sergei is very distrustful of Yoni and his intentions because of the fearful environment he was raised in.
"The one that looked straight into the camera and said, if he ever found a talking goldfish he wouldn't ask of it a single thing." Sergei talks to Yoni and allows himself to be filmed. This shows that he is becoming less fearful of strangers. Also, he answers truly, which shows that he is trying to be more trusting of others.