3rd assistant director The 3rd assistant director — variously , the 3rd AD or the third — supports the work of the 1st and 2nd assistant director s . The third is also the assistant director who works most closely with runner s – particularly with some types of production runner on location . In particular , they are responsible for the direction of background activity in a shot , as with extra s or moving vehicles . As seen in " All About the Girl " , an episode from the fifth series of " Doctor Who Confidential " , a substantial amount of the third ' s time is spent working with extras . In the filming of " The Beast Below " , for example , third Heddi - Joy Taylor - Welch was seen providing actual direction to extras by giving them motivation for their individual action , and for ensuring that the overall movement of extras created an appropriate mood for the shot . She was also seen , on this occasion , to be responsible for the procurement and control of live animals in shot . This aspect of the job can be challenging , as extras are not required on set for very long during any one particular day , but they are contracted for an entire day . Thus the third must provide for a waiting area for the extras , and to be mindful of their individual progress through the wardrobe and make - up departments . In short , the third must be sure that when the first calls for action , every extra is in place on set doing the required action . Beyond the management of extras , however , the third has some specific requirements on location that are delegated by his or her senior assistant directors . Chief amongst these is " locking the set " ; that is , keeping the members of the public far enough away from the shoot that they do not interfere with the take . In this capacity , the third is somewhat of a crowd control agent , and is often assisted by production runners . They will sometimes have to liaise with civil officials , like the police , who have a public duty to control crowds . Conversely , the third may also act as a conduit for principal actors to meet the public who have shown up to a location shoot , during those moments where that actor is not actively required for filming . In short , the third is sometimes the most direct point of contact between the public and the " Doctor Who " production team during location filming . Thirds are also messengers , especially on location shoots where the distance between actors and cameras may be more extreme . Although there was no actual third on " The End of Time " part two , an example of this job functionality was clearly captured in the " David Tennant Video Diary " series . During the filming of " Mickey and Martha scene " , Tennant was quite physically remote to the positions of director Euros Lyn and 1st AD Peter Bennett . Thus instructions had to be relayed via a walkie - talkie , likely belonging to 2nd AD , James DeHaviland . Tennant at one point grabs the walkie - talkie for a direct conversation with the two directors about his position and movement in the scene . At the conclusion of the conversation , Bennett can be heard to remark , " You ' d make a good assistant director , " and the two joke about how Tennant might have to take up that work , since he ' s about to lose his job on " Doctor Who " . Thus , assistant directors can be seen to be " relay stations " of information , radiating out from the physical position of the director .