Christa B. Allen on Jennifer Garner Friendship and How TikTok Is Empowering Her (Exclusive)

Allen has played a younger version of Garner twice on screen and also has almost 2 million followers on TikTok.

Christa B. Allen is opening up about her unique bond with Jennifer Garner and her popularity on TikTok. The actress played the younger version of Garner's characters in both 13 Going on 30 (Jenna Rink) and Ghosts of Girlfriends Past (Jenny Perotti), and talked to ET's Deidre Behar about having Garner as a mentor while promoting her new Hallmark Movies & Mysteries holiday flick, Christmas for Keeps.

Last month, Allen celebrated her 30th birthday -- a key plot point in 13 Going on 30 -- and Garner shared a sweet shout-out to her on Instagram, writing, "Look who's 30! The OG Jenna Rink! Happy birthday, @christaallen!"

Allen definitely appreciated the gesture.

"Well, first of all, just the fact that Jennifer would take the time meant so much to me and Madeleine Stowe [whom she worked with on ABC's Revenge] made me a message as well," she tells ET. "And some of these people have been mentors to me in certain ways, even if it's not a direct conversation, I watched the way these women move in the world with such grace. Madeleine Stowe was one of the best teachers I could have ever had on Revenge, just watching the way -- not only the way she would work as an actor, she's just brimming with talent, but also grace -- and the way that she always fought for what was right for people around her. And Jennifer's the same way. Like, I've just learned from watching her."

Allen said that one memory that has stuck with her is Garner's kindness on the set of 2004's 13 Going on 30.

"I remember being stunned when I made the movie at just how kind she was, she's this incredible movie star, but she's also just a really real kind person," she notes. "And people often ask questions about what movie stars are really like. And I often find that they're as amazing as you think they are. And Jennifer's certainly one of them."

She revealed it was actually Garner who recommended her for her role in 2009's Ghosts of Girlfriends Past.

"I will just say that me playing her in Ghosts of Girlfriends Past was a function of Jennifer speaking up, they were looking to cast the role and they were like, 'We don't know what to do. We're trying to find a younger you,' and Jen was like, 'Call up Christa. She's done this before and she's good at it,'" she recalls. "So I really owe that role to her as well. And that's again, just speaking to her kindness."

Allen has had fun with playing Jenna Rink on TikTok, and even took it one step further by putting out a short video series titled "Vienna" about turning 30 and meeting her 13-year-old self. 

"I never expected things to take off in quite the way that they did when I started posting 13 Going on 30 inspired content on TikTok," she admits. "I was just having fun, ripping on ideas. And then I saw that people really, really enjoyed it. So I kept doing it because I saw the joy it was bringing to people. And then of course I had to acknowledge everything coming full circle with my 30th."

Allen is extremely popular on TikTok and has almost two million followers.

"I found that when I started using TikTok, it felt like millions of us were in on the same inside joke and that feeling of community really lit a fire in me, and it's still the way I feel about TikTok," she says about why she enjoys the platform so much. "I feel like TikTok is like the home for misfit toys or whatever it is. I feel like it's where all the people who didn't feel cool enough for Instagram, we all went to TikTok and now we're just there vibing, and I love it. I love it so much."

TikTok has also given her the power to create her own projects the way she wants to do them.

"There is an unspoken expectation that as an actor you're meant to be like a child," she explains. "Don't speak until spoken to. You're supposed to sit by and wait for the phone to ring. And before then, you don't have an opinion nor a personality. And for me, it just wasn't working out. I had too much to say, and I love being a part of the community too much. I love participating. I love feeling things with people and making people feel things and talking about it with them. I couldn't just sit by any longer and just wait for the phone to ring."

"And I also think that people have this perspective on social media where it's easy to be like, 'Oh, that doesn't matter. It's social media. It's trivial. It's dumb,'" she continues. "But the truth of the matter is it's just media. Take the word social off, it's media. And if you understand media, you have a better understanding of the world, and how people work. And so, I've just never seen it as this thing to be dismissed. I embraced it and I will continue to, going forward. Just being able to communicate with people in a direct way, without waiting to be given permission to speak, I think, is so cool." 

Allen's latest project is Christmas for Keeps, which airs Dec. 18 at 10 p.m. ET/PT on Hallmark Movies and Mysteries. The holiday film is about a close-knit group of childhood friends who return home after 10 years to take part in the celebration of life of their beloved high school teacher. Allen called the experience filming in Vancouver "magical." She also shared just how dedicated Hallmark is to making their beloved holiday films.

"What really stunned me on the filming the Hallmark movie is just how committed they are to making the snow perfect in every scene," she said. "There's a team of like 10 people who are just snow anglers. ... So just imagine like six guys with wheelbarrows going around with snow and they really paint the scenery. I mean, it's artistry the way they do it."

As for the message of the film, Allen shared that it's about "the love of one's hometown and staying together, but also that sense of leadership and how it can inspire those around you."