(written from a Production point of view)
Richard Arnold (28 April 1954 – 26 January 2021; age 66) was a research consultant on Star Trek: The Next Generation, holding the position title of "Star Trek Archivist", an appointment made not long after the premiere of Star Trek IV: The Voyage Home. As part of his duties, Arnold vetted proposals and final manuscripts for licensed tie-ins (novels, comics, guidebooks, video cover art, etc.) on behalf of Star Trek creator Gene Roddenberry, until Roddenberry died in 1991.
Arnold first met Roddenberry while attending the first Star Trek convention in New York in January 1972. He moved to Los Angeles and became a volunteer tour guide at Paramount Studios (from about the time of Star Trek: The Motion Picture), and greatly impressed Roddenberry with his memory for Star Trek trivia. He often assisted Roddenberry with a range of Star Trek-related duties.
Arnold's vetting became infamous for a series of arbitrary and counter-intuitive decrees restricting novels, comic books,   and games and technical developments, resulting in the nickname "Melakon". 
Arnold himself however said that it was never his job alone to go through scripts, that the work was done with Roddenberry and that when writers, unused to being edited, would become discontent they targeted Arnold instead of Roddenberry since going after Roddenberry would turn the fans against them. 
A fan extra, one of the Enterprise crewmembers, in Star Trek: The Motion Picture, Arnold was also interviewed in the Roger Nygard/Denise Crosby documentaries, Trekkies and Trekkies 2. For many years, he wrote a regular column for the Star Trek: Communicator (magazine of the now defunct Star Trek: The Official Fan Club) in which he answered fans' questions about the shows and movies.
Arnold later appeared as a background performer, portraying a Romulan crewmember aboard the Narada in 2009's Star Trek. In the trading card set released for Star Trek Into Darkness, the subsequent film, he was one of those who signed collectible autograph cards.