Recently, a photo from the production of the upcoming second season of Big Little Liesshowed the Wrinkle in Time actress angrily winding up to hurl a frozen confection at the back of Streep's head -- and the actress told ET that it was one of the best professional moments of her life.
"It's a Top 5 moment in my career, for sure," Witherspoon told ET's Brooke Anderson at the premiere of her new docu-series Shine On With Reese in Hollywood on Monday.
That's a big statement coming from a woman whose storied career includes a score of beloved characters, blockbuster movies and an Academy Award.
Streep joined Big Little Lies for the HBO drama's hotly anticipated second season, playing the mother of the late Perry Wright (Alexander Skarsgard) and mother-in-law to Celeste Wright (Nicole Kidman), who shows up to town after the death of her son.
However, before new episodes of Big Little Lies premiere sometime next year, Witherspoon's new series, Shine On With Reese, is currently airing on DirecTV's Hello Sunshine channel. The show features one-on-one interviews between Witherspoon and a number of different female superstar guests who open up about their careers and how they achieved their dreams.
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Meryl Streep Says 'Big Little Lies' Cast Enjoys Lots of 'Drunken Meals Together' (Exclusive)
The show's first guest was country music icon Dolly Parton, and Witherspoon dished on how sitting down across from the beloved "Jolene" songstress was particularly intimidating.
"Talking to Dolly Parton was really terrifying. I was so scared," Witherspoon shared. "She's been an idol of mine forever but it was great to be able to highlight women I've respected -- and I love her so much -- and to bring those sort of intimate conversations that we have just as friends to the people, so I was happy to do it."
"I wasn’t scared of a lot of things but I'm a little scared of dolly just cause I didn’t wanna mess it up," she added. "She’s amazing and she was being so open and really letting me into her house and her closet and so it was a big opportunity… She’s really just as genuine and sweet as she seems."
Ava DuVernay, Pink, America Ferrera and Kacey Musgraves are among a few of the other celebrities who sat down with Witherspoon for her new series.