Bill Tanner is a fictional male Human spy in the James Bond, 007 series. The character was portrayed on screen by Michael Goodliffe, James Villers, Michael Kitchen, Rory Kinnear. Bill Tanner is 'M' Chief of Staff, a character who appears infrequently in films and in the books. Bill Tanner is a strong advocate of James Bond, someone who will defend his friend whenever it is needed. Bill Tanners biggest appearance is in 'For Your Eyes Only' as the actor who had played 'M' had just died and they thought it was too disrespectful to cast someone in the role of 'M' so soon after the actor death. Unlike Bond, Tanner does not go on missions. His character appeared every so often but since Quantum of Solace, he character has become regularly. Although at the time of writing, he's only appeared in the two films, it is a strong possibility that his character will appear as much as Miss Moneypenny does.
In the books, Bill is the only one friend that James Bond can truly rely on, the others just see James as being a cold-hearted assassin and want nothing to do with him. In the updated Bond films with Daniel Craig, he might've come across as a secretary to M, his brief later moves away from just being a "P.A." to providing Bond with help and assistance. Although he occassionally appeared in the films, since Rory Kinnear took over the character, his character has become a regular.