i found silence in the library/forest of the dead very very creepy,and the first episode to have that sort of effect on me,maybe because of the shadows and soundtrack...also there weren't any visuals for the Veshta Nerada which made them more real and petrifying. Sky(lesly sharp)really unnerved me in Midnight...she was really freaky and (again)i was put on edge...and with no CGI monster present it made it more scary.
this showed me how the power of acting can actually work to make an impression on its audience and it shows,i hope the Moffat gets an award for his magnificent two parter(Donna in the parallel world really worked well too and showed off Tates acting...which was fantastic!!)it felt like so many plotlines were all twisted in a great yarn of a complex story which is why i love it...although i feel that is overshadowed by Russel's final three episodes(which also deserve great recognition and praise)it was an extremely effective tale.
what storyline of Doctor Who really put you on edge(old or new)?