They got some supporting cast from Roseanne in the early seasons, too. Sarah Gilbert popped up a few times, but Laurie Metcalf just hit it out of the park as Sheldon’s mother, a part that could have been just a one-note caricature (and a nasty one at that) and made it a more sympathetic and believable figure. (Leonard’s mother, however, is still 1-D, and played for laughs, which are few and far between.)
I absolutely love when Laurie Metcalf shows up on BBT. She is awesome.
Although she was not in this particular episode, the one where Penny agrees to give Sheldon acting lessons to help him with his lectures is classic. I couldn’t stop laughing.