It would have been very unusual for a working-class man to be the dominant character in a room full of the wealthy upper class, indicating there is something different about this mysterious inspector
The inspector says his evidence comes from a letter and a sort of diary found in Eva Smith's room, but this is a somewhat plausible reason for him to have all this information
He is trying to show the audience that in order for real change to come about in society, people have to hold themselves to a higher moral standard even if they don't have to legally take accountability for their actions
Shares similarities with medieval morality plays, with the inspector representing a figure of virtue and the Birlings representing vices like greed, pride, envy, and lust
Inspector Ghoul: '"We are members of one body, we are responsible for each other, and I tell you that the time will soon come when if men will not learn that lesson, then they will be taught it in Fire and Blood and anguish."'
Priestley's opinions and the inspector's message can also be seen as feminist, as the person who dies is a woman and the inspector blames society for their treatment of her