The anecdote is recounted in “Jimmy Stewart: A Life in Film” by Roy Pickard. According to Pickard, Capra was unimpressed with Stewart’s initial delivery of Smith’s closing line: “You think I’m licked. Well, I’m not licked. I’m going to stay right here and fight for this lost cause. Somebody will listen to me.” Capra thought Stewart sounded like, “an actor trying to put on a voice with a rasp.”
So, when shooting wrapped for the day, Stewart went to a doctor and asked if the man could make his throat sore. As Stewart recalled, this was the doctor’s dumbfounded response:
“I’ve heard you Hollywood folk are kinda crazy, but you take the cake. You want me to give you a sore throat. It’s taken me twenty-five years of study and practice to keep people from getting sore throats. Now you want me to give you one… OK, I’ll give you the sorest throat you’ve ever had.”
To do so, the doctor chose mercury dichloride, dropping it down Stewart’s mouth to produce a raspy throat. He even accompanied Stewart to shooting the next day, ready to make his throat scratchy again should the mercury wear off.