Robert Arthur Knox was an English actor who portrayed the character of Marcus Belby in the film Harry Potter and the Half-Blood Prince, and had signed to appear in its sequel Harry Potter and the Deathly Hallows – Part 1.

Harry Potter and the Half-Blood Prince is a 2009 fantasy film directed by David Yates from a screenplay by Steve Kloves, based on the 2005 novel of the same name by J. K. Rowling. It is the sequel to Harry Potter and the Order of the Phoenix (2007) and the sixth instalment in the Harry Potter film series. It stars Daniel Radcliffe as Harry Potter, alongside Rupert Grint and Emma Watson as Harry's best friends Ron Weasley and Hermione Granger respectively. The story follows Harry's sixth year at Hogwarts as he receives a mysterious textbook, falls in love, and attempts to retrieve a memory that holds the key to Lord Voldemort's downfall.