Movies:
* The King's Speech (2010) - King George V
* The Boat That Rocked (2009) - The Count
* Four Weddings and a Funeral (1994) - Gareth
* The Lost Boys (1987) - Max
* The Importance of Being Earnest (2002) - Algernon Moncrieff
* The Girl in the Cafe (2005) - Simon
* The Inbetweeners Movie (2011) - Mr. Brown
* The Borrowers (1997) - Homily Hogg
* An Ideal Husband (1999) - Lord Goring
* The Madness of King George (1994) - Dr. Willis
* The Wind in the Willows (2006) - Mr. Toad
* The Secret Garden (1993) - Archibald Craven
Television:
* The Crown (2016-2020) - Lord Mountbatten
* The Vicar of Dibley (1994-2007) - Hugo Horton
* Doc Martin (2004-2022) - Bert Large
* Green Wing (2004-2006) - Dr. Gordon
* Episodes (2011-2017) - Merlin
* The Thick of It (2005-2012) - Peter Mannion
* The Worst Witch (2017-2020) - Mr. Hogg
* Call the Midwife (2012-present) - Dr. Reginald Turner
* Midsomer Murders (1997-present) - Various roles
* Poirot (1989-2013) - Various roles
* Being Human (2008-2013) - Various roles
Other:
* The Adventures of Tintin (2011) - Voice of Thomson (animated film)
This list represents just a fraction of his extensive filmography. Haig has also performed in many theatre productions throughout his career. To get a full list of all his works, you can visit his IMDB page.