During the first scene at the dinner table, Arthur Birling says "Steady, the Buffs". This phrase means "stay calm, be careful, and persevere", and is associated with the 3rd Regiment of Foot (The East Kent Regiment), whose nickname was 'The Buffs'. The phrase is thought to have arisen when the Regiment was stationed in Malta in 1858, and was popularised in Rudyard Kipling's novel, "Soldiers Three". 'Buffs' refers the colour of the facings worn by the regiment, starting in the 18th Century.
The trivia item below may give away important plot points.
In the stage play on which this film was based, the inspector's name was Goole. It was changed in the film to hide the twist at the end of the film when it becomes clear that the inspector does not really exist and is a figment of everyone's conscience.