In the 2006 Doctor Who Christmas Special, The Runaway Bride, over a period of months Lance, who is working for the Racnoss doses Donna with Huon energy in order to attract the Children of the Racnoss out from the centre of the Earth. The Doctor helps Donna escape. During this time, as Wikipedia succinctly describes;
Lance reveals he was working for the Empress and purposely fed huon particles to Donna to help free the Empress' children. Donna and the Doctor escape, and the Empress decides to use Lance as a substitute, force-feeding him huon particles and then throwing him into the pit.
Is there a reason given for the elaborate dosing of Donna over a period of months instead? If the subsititute can be force fed a batch and chucked into the pit in a matter of seconds why concoct this long running scheme that culminates in going along with a wedding before being discarded at the last moment? Why not just pick a random off the street and activate the plan in seconds?