You might know Laura Osnes as “Small Town Sandy,” the fresh-faced ingénue from Minnesota who (along with Max Crumm) won the reality TV casting competition Grease: You’re the One That I Want! She went on to make her Broadway debut as the hopelessly devoted Sandy Dumbrowski in Grease and won the hearts of a gazillion fans in the process. Now Broadway’s young sweetheart is growing into a full-fledged leading lady as she takes over the role of Nellie Forbush from Tony-nominated star Kelli O’Hara (who is out on maternity leave until October) in Lincoln Center’s award-winning revival of South Pacific. In her first week of performances, we got an exclusive behind-the-scenes peek at Laura in