The 29-year-old Australian actress plays Kayla Pospisil in the new movie, a composite character based on young women who worked at right-wing news network Fox News, who were allegedly sexually harassed by network chief Roger Ailes.
However, as an Aussie she did not understand her character's politics when she first landed the role, and so did some secret social media sleuthing with a fake Twitter account to work out what motivates "young millennial conservative girls".
"I didn't understand her to begin with," Robbie tells Variety. "But my process is to do a ton of research, consider every single option, know every single situation, scenario, thought and motivation inside and out, so I can step onto set and then let it all go."
Robbie's performance has earned Golden Globe and Screen Actors Guild Awards nominations, and has impressed her castmate Charlize Theron, who plays Fox News' former star host Megyn Kelly, who clashes with 'Kayla' in one scene over remaining silent for so long about Ailes.
"I think her (Robbie's) performance in this movie is a very rare performance," Theron adds. "I've seen this movie 50 times now, if not more, and every single time she gets me. It's just ridiculous - and I'm dead inside! And she gets me every f**king time."
"Bombshell" is in cinemas now.