Technically, all of these biographical details are indicated in the script. It's Lohan who internalizes these details, and makes Annie and Hallie come alive as wholly realized, individual characters. Lohan's Hallie and Annie are each imbued with subtle ticks that make them easily distinguishable from each other. Take the moment when Annie and Hallie emerge from their rides to camp for the first time. Their reactions are indicative of their personalities: Annie spins around with wonder, and Hallie surveys the camp appraisingly. When Hallie and Annie switch places and visit their other parent's house, these reactions are repeated. By this point, Annie and Hallie have cut their hair and pierced their ears so they’re even more identical. Yet from the way Annie spins and gawks as if the London townhouse lives up to expectations, you know which twin is which.