Dame Helen Mirren said playing a Hollywood gossip columnist has made her less sympathetic towards the press.
The 70-year-old Oscar winner, who portrays actress turned journalist Hedda Hopper in Trumbo, admitted at a press conference she is "scared to s***" by celebrity journalists.
"In our business, we have to be open to the press. It is an essential part of the job. But at the same time, (I feel like) I'm about to fall off the wire into the pit of snarling snakes," she said.
Hopper - who was accused by her critics of viciously targeting those whose opinions or lifestyles she did not agree with - had abused her power, Dame Helen said as she attended the film's European premiere in London's Leicester Square.
"Hedda abused her power as I think it's quite the temptation for people in the business to do just that," she said.
"At times we all get caught up in our own hubris as Hedda did and the end result of that is she found herself on the wrong side of history."
Asked if the role had made her more sympathetic to the press, she added: "It's rather the opposite actually."
The actress - who wore a pink dress-coat by Suzannah paired with a Rachel Black hat to the BFI London Film Festival screening - said Hopper did have some redeeming qualities.
"She was an interesting person because she was very powerful at a time when women weren't very powerful and she did it all by herself.
"There was a lot to admire in her and it was interesting researching how she got into that position of power."
Bryan Cranston stars as Hollywood screenwriter James Dalton Trumbo who was blacklisted because of his support for the Communist Party.
The 59-year-old actor, best known for playing meth maker Walter White in award-winning series Breaking Bad, said there is added pressure in portraying a real-life person, particularly when their family is still alive.
He said: "Any time you play a non-fictional character there is an extra level of responsibility. Dalton Trumbo's daughters are alive and know their father very well and while you don't want to get caught doing an impersonation, you want to be able to get a sense and essence of him.
"They (his daughters) are a great test because they'll see the movie and say whether or not that's their father. Fortunately they both blessed the film."
Jay Roach's biopic of the Roman Holiday writer marks the second collaboration between Mirren and Cranston following Love Ranch in 2010.
Cranston said: "The first time I met her was at 5.30am on a make-up chair on a Monday, dreary-eyed, tired and she doesn't have a stitch of make-up, she just got up and she asked me about my weekend."
He described her as a "superb actor".
Mirren returned the praise, saying: "I'm a huge fan of Breaking Bad and an enormous fan of Bryan Cranston, not just in the show but also in the theatre. He is one of the pre-eminent American actors."
The pair were joined on the red carpet by co-stars John Goodman and Adewale Akinnuoye-Agbaje.
Trumbo opens in UK cinemas on February 5 2016.