Jamie Knox said his brother was "upbeat" before the attack
The younger brother of a teenage actor has described to an Old Bailey jury the moments before his sibling was stabbed to death outside a bar.
Rob Knox, 18, who had a part in the latest Harry Potter film, was stabbed five times outside the Metro Bar in Sidcup, south-east London, in May 2008.
Jamie Knox, 17, said his brother was "upbeat", having just finished work on Harry Potter and the Half-Blood Prince.
Karl Bishop, of Sidcup, denies murder, saying he acted in self-defence.
Mr Bishop, 21, of Carlton Road, also denies five charges of wounding with intent to cause grievous bodily harm to five other youths.
Jamie, from Swanley, Kent, told the court that his brother confronted Mr Bishop after a row over threats which had been made earlier in the evening.
Recounting the scene outside the bar, Jamie said his brother was "going mad" and shouting: "You held a knife to my brother, why did you hold a knife to my brother?"
He described Mr Bishop as being aggressive.
"He was lunging, sort of swinging his arms, flailing everywhere.
"I believe other people got involved almost instantly. I couldn't believe what I was seeing."
Jamie said he then helped his brother and threw several punches at Mr Bishop, before he ended up being thrown into a flower bed.
Jamie told the court the next thing he saw was his brother being led away by a friend, saying: "I've been stabbed."
He told the jury that his older brother was "very protective" and "always in a happy mood".
Brian Altman QC, prosecuting, asked Jamie if his brother would shy away from trouble or get involved.
He said: "He would but he wouldn't do anything unless he felt it was necessary."
Mr Knox played the character of Marcus Belby in Harry Potter and the Half-Blood Prince and had been looking forward to a role in the next instalment of the series.
The trial continues.