Frank's tank was originally meant to be filled with Helium. Dennis Hopper, the guy who played Frank, turned out to be an experienced drug user and claimed to have insight into Booth's choice of drug, which was most definitely not helium. According to Hopper, the drug in question was amyl nitrite, an angina medication that is used as a recreational inhalant in the disco club scene.
Reflecting on the role some years later, Hopper lamented having made this change, saying that Frank would have been far more bizarre and frightening if, instead of using the gas to get high, he carried the tank around for no other reason than to alter his voice.
Lynch filmed a four-hour movie, which was cut down to one hour fifty-nine minutes by the studio. There's a whole two hours and one minutes worth of missing footage.
Val Kilmer was offered the role of Jeffrey but turned it down, saying that the script he read "was straight-out, hardcore pornography before page 30". Kilmer stated that, if he had been given a copy of the script that Lynch eventually filmed, he would have gladly taken the part.