"The life and soul of the party, he was very outgoing, loved sports, and would always strike up a conversation with people.
"He was respectful to others and adored by all his family and friends. He was asset to the family."
Mr Knox's 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."
Vicky May, 16, a cousin of Mr Knox, said: "You never think it will happen to you. Rob wasn't a thug, it wasn't gang-related. Everyone loved him.
"He was more like a brother, he would always look out for me. He was friends with so many people."
'Shocked and saddened'
Mr Knox had won a part in the new Harry Potter film, The Half Blood Prince.
After the fatal stabbing Warner Bros released a statement which said: "We are all shocked and saddened by this news and at this time our sympathies are with his family."
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, and a 21-year-old was stabbed in the neck. He 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.
Friends of Rob Knox lay flowers at the scene of his death
A spokesman for the Metropolitan Police said: "We believe the stabbings took place in the street, outside the venue.
"We do not believe the incident is gang-related."
He added: "At this early stage we retain an open mind as to any motive, and enquiries continue to establish the full circumstances of the incident."
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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