During the first scene at the dinner table, Eric 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 originated when the Regiment was stationed in Malta in 1858, and was popularised in Rudyard Kipling's novel, "Soldiers Three". 'Buffs' refers the dull yellow colour of the facings worn by the regiment, starting in the 18th Century.
George Cole (Tram Conductor, uncredited) was a protege of Alastair Sim and they appeared in several films together.
The original stage play is set entirely in the Birling house and does not feature any 'flashbacks' to the events being described.
To put a flea in someone's ear means to reprimand him, to send him away with a strong rebuke. This British idiom is derived from a fourteenth century French term, which meant filling someone with sexual desire.