"There were some very talented people on that stage with her, or in the exercise with her," he said. "But I have to say, I couldn't keep my eyes off of her. There was something about the depth, about the size, the capacity, the intensity of what she was doing. And I would look around, and finally my eyes would come back to this woman. She was just reading, reading a book on the sofa. But, the size of her intensity was huge."
In 1975, Posnick directed Streep in a production of "Happy End." She was cast in a small part, "one of the Salvation Army lassies," he said. Then one night, the actress in the leading role fell ill.
"Meryl was not the understudy," said Posnick. "But she stepped in, after an afternoon's rehearsal, and played the play. And I remember standing in the back of the theater in awe that she had absorbed the entire production, absorbed the part. And they're singing songs in there. It's a musical ... where you have to really be present, and she had the whole thing down. And not only that, I watched her invent the character in a different way than she had seen it.
"And of course, in that situation, everybody's very awake and alert. And everybody's feeling her, energy-wise and, life-wise. And so she just swoons to triumph."
Posnick said that he still is amazed today watching Streep on the big screen.
"I sit in amazement. I just sit in amazement," he said, "at her capacities, and her abilities, and her gifts and her generosity. I think of all the things that she gives as an actress, she gives fully, all the time. She's always there, always present, great intellect and great heart as well."
Success is never foretold, but for those who knew her when she was young, Meryl Streep seemed always to be on a path to greatness. Posnick said her future as an actress was clear.
"Oh, crystal clear. I think -- I don't know if she had a choice," Posnick said. "That was the direction. There was no equivocation. Just as onstage and in film, there's no equivocation. Everything's crystal clear. And that was her then, as well, from that moment on the sofa, it was crystal clear, there was no mistake, there was no ambiguity. She knew what she was doing, and she knew what she wanted to do. So, I really think in terms of her career, I think she was aimed like an arrow and was destined to do this."
Castrilli said she will be watching the Oscars next week and rooting for Streep. It has been a few years since the two last talked, she said. But she doesn't doubt that her old friend is the same as ever: Meryl Streep has always been Meryl Streep.
"I mean when you're starting out, you know, you make a few mistakes," said Castrilli. "But, you know, the term 'perfect' comes up a lot with Meryl whenever anybody talks about her. It's like, 'Oh, she's perfect. She's got the perfect hair. She's got the perfect cheekbones. She's got the perfect talent. She's just perfect.'
"And you know, it's not so much that she's perfect. It's that she's real. She's like a real honest-to-goodness person that you trust. ... She has everything going for her, and I think it's because of that honest-to-goodness realness that she seems so perfect. "Obviously nobody is perfect," she said, "except maybe Meryl."
Watch "Before They Were Famous," on a special edition of "20/20" Tuesday, March 2 at 10 p.m. ET