ET's Kevin Frazier caught up with the Love, Simon star at the STX Entertainment presentation during CinemaCon 2018 at The Colosseum at Caesars Palace in Las Vegas on Tuesday, where she opened up about the thought process that goes into her entertaining Instagram posts.
"Everything is so fly by the seat of my pants," Garner shared. "I just say to my awesome buddy, partner-in-crime, assistant, social media guru, Mo, I just say, 'You know what would be funny? I wanna do me cleaning out this bag,' and she'll be like, 'Ah, ok.' And then she does it."
"She's so good at editing, I said, 'How do you decide, when you're editing, what to do?' And she said, 'I just make fun of you. Because we're so close that I just pick out whatever is making fun of you.' And then we put it up," she explained.
On April 9, Garner actually did post a video exploring what is in her "favorite backpack" after drinking some wine. As it turns out, she's got a date book, heart-shaped stickers, a purple geode rock, more heart stickers, a notebook, a bunch of colored pens, an entire roll of bigger heart stickers, some tea bags, index cards, and more heart stickers.
Some of Garner's other posts are a bit more intricate -- although just as hilariously eccentric -- like when she wished friend Reese Witherspoon a happy birthday by dressing up in full marching band gear and playing the saxophone in the backyard. Turns out, she's actually pretty good at it too.
Recently, Garner celebrated her own 46th birthday, and her 9-year-old daughter, Seraphina, weaved a massive pastel scarf as a present -- and the adoring mom wore it with pride in a photo she posted to Instagram a few days after her special day.
"When your nine-year-old weaves a 12ft long scarf on a peg loom for your birthday present, you’d better #werk." Garner wrote.
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Garner attended CinemaCon to support and promote her upcoming action thriller, Peppermint, in which she plays a woman whose husband and daughter are murdered in a drive-by. When she awakes from her coma, she dedicates her life to learning how to fight and kill, then seeks out those who took everything from her.
While Garner has mostly appeared in grounded dramas and family comedies in recent years, she's certainly no stranger to action -- including her turn as Elektra in 2003's Daredevil and 2005's Elektra, and her five-season run on J.J. Abrams' ABC spy thriller Alias, which came to an end in 2006.
"It felt great, actually, to be back with a stunt team again," Garner shared. "My stunt double and I have worked together since I think the third or fourth episode of Alias… So she's like my sister. So [its been great] for the two of us to get to be together every day, to train together, to go back to all of that."