Judd Hirsch plays Hofstadter
Leonard's anthropologist father is to be played by Oscar-winning actor Judd Hirsch in an upcoming episode of The Big Bang Theory. Galecki suggested the actor for the role after loving him in 'Taxi' and using the actors performance in his preparation for role of Leonard.
“I ran up to him and just kind of bombarded him with verbalization,” said Galecki. “I did share with him, and this is true, that when we were shooting the pilot I watched the first two or three seasons [of ‘Taxi’]. Not because I wanted to base Leonard on his character, but the purpose that his character served in this group of people. [He was] just as much, at times, a voice of reason as he was just as confused as everyone else.” he said.
As yet its not clear when he will appear so maybe we'll see him in the finale.
Personally I don't see any of Rieger in Leonard.