From joe-armstrong.net:
Joe Armstrong plays Hotspur in Henry IV Part 1, and his father Alun Armstrong plays Hotspur’s father, the Earl of Northumberland. In a recent interview with The One Show, Alun Armstrong joked about having the smaller role and said: “Working with my son, it was sort of nerve-wracking at first, but he was just terrific and it was just wonderful to stand there in … his scenes.”