7. Viggo Mortensen - Eastern Promises (2007)
Back when he played Aragorn to perfection in Peter Jackson's The Lord of the Rings trilogy, Viggo Mortensen went all "method" and decided to inhabit the skin of his character for most of the production, which meant learning Aragorn-y skills and taking his sword everywhere with him (and we mean everywhere). Though that in itself is impressive, it wasn't exactly "research," which is precisely what the underrated actor delved into prior to shooting David Cronenberg's excellent crime thriller Eastern Promises. And Viggo is one of the main reasons as to why the movie itself is so good.
Starring as Nikolai Luzhin, a cleaner and driver in the Russian mafia, Mortensen took the prep for his role super seriously - and it's all up there on the screen. To research playing a Russian, he moved to the country for a brief period, and travelled the Urals. There, he immersed himself in Russian life, watched Russian movies and television shows, and even tested his knowledge on the language (he attempted to learn it prior to leaving home, and can speak it fairly fluently now).
He also read books by famous Russian authors, such as Dostoyevsky and Nabokov, and spent weeks researching Russian gang culture with the aid of a professional criminologist. Then there's the subject of the many tattoos Mortensen's character has etched all over his body.
Although the actor opted to use fake tattoos for the movie, he apparently became his character to such an extent that, upon walking into a London-based Russian diner, the entire place fell into silence out of genuine fear. Now that's research.