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Back in 2004, Jennifer Garner had never starred in a movie. Sure, she was the lead in a TV show called Alias, playing a tough-as-nails spy named Sydney Bristowe, but she wasn't a movie star yet. That all changed with a funny little film called 13 Going On 30, where Garner played Jenna Rink, a girl who made a birthday wish that magically came true. Thanks to some wishing dust, Jenna skips her awkward adolescence and goes straight from 13 to 30, waking up as a full-grown adult with a dream job, dream apartment, and dream boyfriend. The one thing she doesn't have though is her childhood friend Matt (Mark Ruffalo) and when she sets out to find him, everything changes.

The film turned Garner into a movie star, but she's not the only one to make it big from 13 Going on 30. The film is filled with famous faces and it's fun to watch to try and spot the celebrities before they were huge stars. There's even an Oscar winner in the cast! It just goes to show that no matter how many times you've seen the film, there are still a few things to learn about it. Here are 13 things you may have never noticed or known about 13 Going on 30:

Before Jennifer Garner was cast, Renée Zellweger was considered for the role of Jenna.

Sean Marquette, who played a young version of Mark Ruffalo's character, Matt, had to wear a fat suit for filming.

In the film, 12-year old Christa B. Allen played a young Jennifer Garner. She had never acted before, but she was hired anyway, because she looked exactly like Garner as a real life teenager.

Allen was such a dead-ringer for a young Jennifer Garner, that she also played the younger version of the actress in the 2009 rom-com, Ghosts of Girlfriends Past.

Jenna turns 30 exactly 13 minutes into the movie.

Garner actually picked the film's director, Gary Winick, after seeing his film Tadpole. It was her first leading role and his first big movie.

Mark Ruffalo almost turned down the role because he was so nervous about the dance scene where he and Garner bust a move to Michael Jackson's "Thriller." To convince him to take the part, the producers hired a dance coach to teach him the moves.

Garner and Ruffalo had such natural chemistry that many of their scenes together were improvised.

The Six Chicks—the Mean Girls-esque clique at Jenna's high school—included Ashley Benson, who went on to appear in Pretty Little Liars, and Oscar winning actress Brie Larson, who starred in Room.

In a deleted scene, Jenna goes to the doctor and the doctor compliments her on her tattoo. Of course, Jenna has no idea that she has a tattoo, so as soon as the doctor leaves the room, she checks her whole body for it.

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When 13-year old Jenna is reading Poise magazine, the issue features a photo shoot of the apartment, that she lives in when she is 30. That's not the only Poise play: Jennifer Lopez is featured on the covers of both Sparkle and Poise magazines. At the time, J. Lo was in a serious relationship with Ben Affleck, Jennifer Garner's future husband. That's not all:

13-year-old Jenna has a photo of Madonna in her bedroom, when she becomes 30-years old Jenna, the photo reappears, but now it is signed by the singer.

Jenna's boss, Richard, is perhaps the most famous actor in the movie. He is played by Andy Serkis and while he's not exactly a household name, he has starred in some of the biggest blockbusters of all-time, including King Kong, The Rise of the Planet of the Apes, and most famously, he played Gollum in the Lord of the Rings trilogy and The Hobbit.