Alun Armstrong (Northumberland) and Joe Armstrong (Hotspur) playing father and son are actually father and son.
Niamh Cusack (Lady Northumberland) is Jeremy Irons' (Henry IV) sister-in-law.
In Henry IV, Part 2, there is bad blood between Prince Hal and the Lord Chief Justice and references to the Justice having imprisoned the Prince for striking the Justice for trying to prosecute one of Hal's friends. The incident that is referred to is not included in Shakespeare's plays, but was probably familiar to his audience because they had been able to see it recently in a play called "The Famous Victories of Henry V". For a modern audience however, the references can be confusing. In his 2014 production of the Henry IV plays with the Royal Shakespeare Company, Gregory Doran made the somewhat controversial choice of taking lines from "Famous Victories" and placing them in Henry IV, Part 1. In these lines, the Lord Chief Justice comes to the tavern along with the sheriff after the Gadshill robbery and attempts to arrest Bardolph, at which point Hal strikes him.