Karl Bishop, who was found guilty Wednesday, could face life in prison when sentenced for the stabbing death of 18-year-old Knox.
Prosecutors say Bishop killed Knox outside a bar in east London in May after Knox stepped in to protect his brother, who had been threatened by the 22-year-old.
Knox had just wrapped up filming on "Harry Potter And The Half-Blood Prince." The movie is the sixth in the Harry Potter series and is due out this summer.
Knox played the part of Marcus Belby, a minor character.