I tend to give a lot of credit to any actor who can embrace their inner child to actually play a child, and play it well. Robin Williams tends to come to mind when he pulls it off in 'Jack' (underrated, but perhaps needs a revisit), but the only man who arguably did it better was Tom Hanks in 'Big' - an Oscar nominated performance from 1988 that has yet to be topped. He nails everything from the way he talks to his subtle mannerisms. It's very evident that Tom Hanks had a lot of fun with this role, and did his homework in some way before pulling it off.
Josh (Hanks/David Moscow) is an average boy living an average life. He hangs out with his best friend, Billy (Jared Rushton), and has a crush on a girl named Cynthia (Kimberlee M. Davis). In a line-up for a carnival ride, he attempts to try to break the ice with Cynthia, but he's told he's not tall enough for the ride. In his frustration, he wanders over to a Zoltar fortune telling machine that "grants wishes". He wishes he was "big", and the next day, he wakes up as a full-grown adult.
Freaking out his own mother (Mercedes Ruehl), Josh realizes he can't go home to figure things out. Instead, he seeks Billy's help, which eventually leads them to New York City, where he can lay low until he can find another Zoltar machine to hopefully reverse his wish. This is where he finds out immediately that there's much more to the adult life than meets the eye.
In killing his time, he manages to find a data entry job for the famous FAO Schwartz toy store. Here, he develops a few new relationships where his adulthood is often tested. His boss, MacMillan (Robert Loggia), sees him as the perfect man child to test the company's toys. The toy making team-leader, Paul Davenport (John Heard) has a strong dislike for him, as he seems to constantly undermine him. Finally, his coworker, Susan (Elizabeth Perkins) likes him and his open mind, but he's still 13 on the inside, and not quite in touch with those things yet.
As the story unfolds, it's a great case of Josh learning a lot about what it means to be an adult, but it equally covers all of these adults and how they would typically deal with a kid's mind. MacMillan embraces it, especially as a toy store owner, Paul is obviously very annoyed by it, and Susan, though it's not quite as obvious in her situation, kinda meets it in the middle. She finds his child-like mind frustrating at times, but that frustration he causes is all part of his charm.
This is perfect for any kid going through those difficult stages of puberty. I remember it well. We wanted to do what adults could do (in more ways than one), and in many ways, this is the film to show them that things aren't necessarily all they're cracked up to be. It's also a good way to show these kids that it's important to embrace your childhood while it lasts. It's a very short time in your life that you might take for granted.