Makers of a Discovery network documentary on presidential chiefs of staff have a perfect participation record.

All 19 living men who had that job gave interviews for "The Presidents' Gatekeepers." The four-hour documentary is expected to air in two parts early this summer.

Filmmakers Jules and Gedeon Naudet (naw-DAY) said Saturday that their well-regarded previous documentary on the Sept. 11 terrorist attack in New York was a calling card that probably helped them convince some former chiefs to participate.

Chris Whipple, formerly of ABC News, and ex-White House photographer David Hume Kennerly are also producers.

Their longest interview was with former Vice President Dick Cheney, who was President Gerald Ford's chief of staff in the 1970s.

Cheney said a chief is more powerful than a vice president.