Mark Williams (born 22 August 1959) is an English actor, comedian, presenter and screenwriter. He first achieved widespread recognition as one of the central performers in the popular BBC sketch show The Fast Show. His film roles include Horace in the 1996 adaptation of 101 Dalmatians and Arthur Weasley in seven of the Harry Potter films. He made recurring appearances as Brian Williams in the BBC television series Doctor Who and as Olaf Petersen in Red Dwarf. Since 2013, Williams has portrayed the title character in the long-running BBC series loosely based on the Father Brown short stories by G. K. Chesterton.