Crook is perhaps best known for playing Gareth Keenan in The Office. He was born in Maidstone, Kent, and grew up in Dartford. Crook began his career as a standup comedian alongside Iain Lee. They appeared at the Edinburgh Fringe Festival in the mid-90's as Chalk and Cheese. Crook's first major television role was as a comedy sketch contributor on Channel 4's The Eleven O'Clock Show in 1998. He was later a member of the main cast of the BBC sketch show TV to Go in 2001.
He portrays Ragetti in the first three Pirates of the Caribbean films, and voices the character in the At World's End video game.
He has appeared in advertisements for Visa and M&Ms (in-character as Ragetti in both). Many of those who have interviewed him have described him as a shy and quietly-spoken man, being also fairly nervous, compared to his role in Merlin (Cornelius Sigan).