A friend who held a stabbed young actor as he died said it was "heartbreaking seeing someone like that go".
Scott Shorter was speaking after his 18-year-old friend, who had a small part in the next Harry Potter film, was stabbed at a bar in Sidcup, London.
He said: "If anyone wanted anything he'd always be there - he's just the best person round here really. Everyone loved him - no-one hated him."
Police said a 21-year-old man had been arrested on suspicion of murder.
Mr Knox and four friends were injured in a fight outside the Metro Bar in Station Road with a man who was armed with two knives.
'Genuinely nice'
Mr Knox is the 14th teenager to die violently in London this year. Police said they were keeping an open mind over his death but they did not believe it was gang-related.
The teenage actor had won a part as one of Harry Potter's fellow pupils in the new film, The Half Blood Prince, which has finished filming.
Film fans have left messages on the Internet Movie Database and friends of Mr Knox have written tributes on the Facebook social networking website. Flowers have been left at the scene of the attack.
Mr Knox's family said in a statement he was "kind and thoughtful" and was "adored by all his family and friends".
His uncle, Kevin May, said: "He was a big guy, always full of fun... a genuinely nice guy, a very caring guy like his family is."
In the incident in the early hours of Saturday a 16-year-old was stabbed in the chest, but his injuries are not life-threatening. A 21-year-old stabbed in the neck is reported to be in a serious condition in hospital.
Officers said another person, aged in their late teens or early 20s, sustained a serious injury to their hand.
A post-mortem examination is expected to take place on Sunday.
The 21-year-old man who is being held on suspicion of murder suffered facial injuries and was arrested after being released from hospital.
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