Rob Knox, 18, died after being stabbed five times outside a bar near London in May 2007 as he tried to protect his younger brother Jamie, 17. Knox had a small part as Marcus Belby in the forthcoming "Harry Potter and the Half-Blood Prince."
Karl Bishop, 22, was found guilty of his murder at London's Old Bailey criminal court.
The court heard Bishop had been involved in a dispute with Knox at the bar the previous week. The following week he returned but was refused entry and ended up in another confrontation with Knox and his friends.
He returned home and armed himself with two kitchen knives. During the altercation Bishop threatened that he "would stab anyone who came near him," prosecutor Brian Altman said.
The court heard that Bishop showed no signs of remorse. When told that Knox had died, he replied: "Yeah, sweet."