Bruno Ganz in Downfall was as chilling a portrayal of an historical figure as you could ever wish to see.
Saying that Meryl Streep in Sophie's Choice was also as moving as I can remember seeing.
Always liked Lawrence Olivier's comment to Hoffman who had been irritating him on set for the filming of Marathon Man, where Hoffman would not sleep for 24 hours to look properly stressed, and go on huge runs to get into character, to which Olivier said 'my dear boy, why don't you just try acting?'.