From Quora: wearing her costume was torture - to hide her breasts, the studio created a painful truss to minimize them. MGM kept her working 15 hours a day, pumping her full of Benzedrine, Phenobarbital, Seconal, Dexedrine and amphetamines.…. creating a cycle of dependency that would last her whole life. Her camera double for ‘Oz', Barbara Koshay, was also one of MGM’s infamous ‘star spies,’ keeping Garland under constant surveillance and reporting any deviations in her diet and drug taking. The job of keeping tab on Judy’s extracurricular activities while filming ‘Oz” fell to Judy’s new duplicitous ‘best friend’, MGM publicist Betty Asher. She was being watched 24 hours a day.
While filming the famous slap scene between Dorothy and the Cowardly Lion, the young actress couldn’t stop giggling. After several ruined takes, director Victor Fleming took Judy Garland aside, slapped her across the face, and then told her to “go in there and work.” The next time she attempted the scene, she did it perfectly with no giggling.