Before he was on television, he played Bob Fossil and was the first choice for the role, however contract limitations between Channel 4 (Garth Marenghi's Darkplace) and the BBC (Mighty Boosh) saw Rich Fulcher take the role permanently, in The Mighty Boosh stage show. He is currently the only regular cast member (of six) from the TV version of Garth Marenghi's Darkplace who has yet to feature in the TV version of The Mighty Boosh. He is the only main character from Darkplace (of four) not to have appeared in The IT Crowd.
Holness first appeared on television as a cast-member of the 2000 BBC Choice TV series Bruiser.
Garth Marenghi transferred to UK television in the 2004 Channel 4 horror comedy Garth Marenghi's Darkplace. Despite critical acclaim and a cult following on the Internet the series suffered from relatively low ratings, and Holness is perhaps better known by TV audiences for his cameo as Simon (the Computer Guy) in series two of The Office. He played a similar character (among others) in the 2000 TV series Bruiser.
Holness's comedy character, Merriman Weir, a guitar-picking folk singer, has performed at small comedy clubs in London and around the UK. Merriman Weir has also appeared as a guest in the pilot episode of the Channel 4 comedy chat show Man to Man with Dean Learner (2005) (then known as Deano's After Dark) where he was interviewed by Garth Marenghi's publicist Dean Learner — a character created by Richard Ayoade.
Following on from the above successful pilot show, Holness and Ayoade recorded a six-part series for Channel 4 entitled Man to Man with Dean Learner in late May and early June at Teddington Studios, which was broadcast on terrestrial television on Friday 20 October 2006 at 11.05pm on Channel 4 and continued for 6 weeks. Holness played Dean Learner's different guests in each episode (although the Merriman Weir episode was recorded first, it was the fourth one to be broadcast in the series).
In 2012 he wrote and directed a one off play for Sky ArtsPlayhouse Presents series entitled 'The Snipist'. It starred Douglas Henshall in the titular role, protecting the citizens of a dystopic alternate 1970s Britain against the threat of rabies. John Hurt provided 'The Voice of The Ministry'
This endlessly entertaining web-series from the PBS Digital Studios gives new life to old interviews, which had previously been sitting unappreciated in the personal archives of journalists, broadcasters and biographers, using the power of animation. Combined with music, collage and a sense for intimate, revealing moments, the series reveals hidden sides of stars as diverse as Jim Morrison, Wilt Chamberlain and David Foster Wallace.
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