The 2nd assistant director — variously, the 2nd AD or the second — is the chief assistant to the 1st AD. He or she is chiefly responsible for the maintenance of the call sheet, and for looking after the principal cast.
In the performance of the first of these duties, the second must liaise with the production office, and particularly the production secretary, who passes the list onto the production office (usually to the production co-ordinator) for approval. Once approved, the secretary finalizes copies of the list. The second then physically provides the cast and crew with these copies, so that everyone involved with a day's shoot knows when and where they're supposed to be. Though it may seem trivial, this action is absolutely vital to the first's chief goal of keeping principal photography moving according the shooting schedule.
As filming progresses on any particular day, the second is busy actually getting the principal actors in place at the appropriate time. This can be a complex matter of organization, as the second must correctly allow for the time other departments — such as wardrobe and make-up — will take in preparing actors for filming. If on location, the seconds have the additional worry of allowing time for the physical transport of actors from their overnight accommodations.
BBC Wales productions have always enjoyed the presence of a 3rd AD, who principally deal with extras. In the special case of the 1996 tele-film with Paul McGann, where there was no third, second David Klohn would have also had to oversee the extras, as well, and otherwise assume the general responsibilities of the third.