Good question, Like you mentioned, Sean Bean(very well known from goldeneye and LOTR), Lena Hedley was known for her terminator work, Peter Dinklage(Elf, Nip/Tuck), and lets not forget Natalie Dormer(playing Anne Boylen on the tudors, a popular TV series a few years before GOT). Basically if you watched popular scifi/historical shows/movies these characters(some) were well known.