In the late '80s, one of the more popular film trends was the body switch comedy. The latest (and arguably greatest) of these is 1988's Big, which starred Hanks as Josh Baskin. A teenager who wishes to be "big," Josh gets more than he bargained for when he wakes up in the body of a 30-year-old man.
The role featured a high degree of difficulty, which is why Hanks received his first Academy Award nomination for this performance. Long removed from his teenage years at the time of filming, Hanks was able to sell the potentially goofy premise by committing himself to the part. Studying David Moscow's ("young" Josh) mannerisms, Hanks accurately portrayed a youth of the times using great comedic timing and the dramatic chops he's known for today.
Mixing child-like enthusiasm with emotional poignancy (while teaching viewers life lessons along the way), the actor is the main reason why this film is so charming and funny.