The film sees twelve-year-old Josh Baskin (David Moscow) make a wish on a fortune teller machine called Zoltar to become "big", only to wake up the next morning as an adult. The "big" version of Josh (Hanks), with the help of his best friend Billy (Jared Rushton), must survive in the adult world until he can manage to transform himself back to his childhood self.
Some quick research into '80s cinema shows that Big's high concept is not an entirely original one for the era, with several other child-to-adult transformation or bodyswap films floating around in the years either side of this film's release. Most of those have since sunk almost without trace, and certainly none have been retained with such acclaim and affection as Big. And it's no exaggeration to lay most of the credit for this firmly at the feet of Hanks.
That's not to say that the film isn't highly successful in other ways. The cast are almost universally strong: Elizabeth Perkins is incredibly likable as disenchanted businesswoman Susan who is enamoured with adult Josh's childish personality, eventually becoming his love interest; John Heard also does well as Josh's antagonistic coworker Paul, doing well to successfully flesh out a character who starts out as a fairly one-dimensional counterpoint to Hanks' hero; and credit must also be extended to Rushton, who demonstrates genuine energy and chemistry with Hanks throughout.
But, as stated before, this is Tom Hanks' show through and through. From the first moment the adult version of Josh is on screen to the last, we believe in him completely. Everything from Hanks' body language to the inflections within his voice are perfect throughout. His performance is a consummate success for many reasons, but possibly the most important of all is that Hanks doesn't play Baskin as a boy in a man's body, he simply plays him as a boy. Hanks becomes a child, he just happens to look about thirty.
The other half of Big's brilliance comes from the writing talents of Gary Ross and Anne Spielberg combined with Penny Marshall's direction. Marshall knows exactly how to make the audience not only understand what Hanks' character is feeling, but feel it themselves. Josh's first night away from home, staying in a complete dive of a hotel in the middle of New York City, is one of the film's most emotional scenes where you feel fear just as palpably as Hanks' character. Probably the film's most enduring and iconic image is that of Hanks performing on an oversized foot-operated keyboard alongside his boss Mr. MacMillan (a charming Robert Loggia). How better to demonstrate that Josh is a child in a grown-up world than by having him play beginners' pieces "Chopsticks" and, of course, "Heart And Soul" on a giant piano?
Big isn't perfect. Like I said, the plot is nothing original and there are a handful of times where you have to remind yourself that this is a modern fairytale to accept some of the lucky breaks that Josh gets in his job and his circumstances. The film also misses a beat slightly at the end of the middle act, before picking up the pace and the interest again as things move towards the finale.
But these minor issues can be overlooked thanks to the charm and magic the film is imbued with in so many other ways. Receiving an Oscar nomination for his performance, Big was the first real opportunity Tom Hanks had to show that he was an actor of great range and adaptability; five years later, with a Best Actor win for Philadelphia, Hanks had demonstrated that his abilities first shown off in Big were no fluke.