Letts and Newbery explained how the caves were not complete, with filming soon to come. To deal with this, the BBC took people off other shows to finish the set.
Letts theorised that CSO could have been used with a Dinosaur puppet instead of the costume that was actually used. Combe tried to keep the suspense of the story going for as long as possible. He did this by not filming the Silurians directly. Combe also had the idea to use three lenses to achieve the "Silurian view" effect.
There were many problems with the costumes. They had no breathing apparatus so intervals had to be taken to allow the actors to cool down. The actors had little experience walking in such a constricting costumes and therefore their walking was impaired.
Combe obtained a helicopter for filming. He used this both in the show and for filming aerial shots. Combe was acquainted with Fulton Mackay (John Quinn), who Jon Pertwee feared would upstage him. Peter Miles' role was also discussed. Miles explained that his acting in the scene where he goes berserk was aided by the fact he had been blamed for holding up filming as he was having his plague make-up applied.
Combe contacted Geoffrey Palmer's agent and asked him where he lived. Knowing that he would have to get off the train at King's Cross, Combe contacted Palmer and told him he would film him coming off the train. This was their first meeting.
Dicks explained that the plague story was added to stop the usual mid-serial sag that was often present in seven-parters. Letts recalled he received a letter from a lawyer complaining that he had given permission for his number plate "Who 1" to be used on Bessie, but had never received any thanks. In reply, Letts explained that a change in producer was the reason for the mistake. He went on to say that every time Bessie was on public roads during filming, they were breaking the law.