Lily Collins met Ted Bundy 's ex-girlfriend Elizabeth Kloepfer to get into character for Extremely Wicked, Shockingly Evil and Vile movie.
Collins, who plays the serial killer's ex-partner in the Ted Bundy movie, said she was "gracious" adding she was thankful for all her help.
Speaking on This Morning Collins said: "I went and met the woman that I play. It was really helpful. She was so gracious, giving me material to look at and speaking to me.
"It was really helpful and and her daughter Molly, were so gracious in inviting me in and giving me material to look at, and just speaking to me and allowing me to ask questions."
She added she wasn't sure if Kloepfer will see the movie.
"I don't know if she's going to see the movie because it's difficult but within the filming process she came on set and she was a positive light on set," Collins said. "You wouldn't expect that with what happened. She gave us her support. She's really lovely."
Zac Efron stars alongside Collins in the Bundy movie which shows the serial killer's home life before his murder spree.
Bundy confessed to the murder of 36 women and was executed in Florida in 1989.
Elizabeth became suspicious of Bundy after five years together and even tried to turn him into the police.
She recorded their time together in the memoir The Phantom Prince: My Life with Ted Bundy, published when she was 36 and Bundy was on death row.
Bundy and Elizabeth dated from 1969 to 1980 when they parted ways for good.
Collins insisted she only looked at crime scene footage Elizabeth would have seen - and no more than that.
"I play the girlfriend, she had a daughter, and they were a family unit before he committed the crimes. In preparation for the film, I wouldn't look at any imagery. I didn't want to look at anything Liz hadn't seen."
Efron, speaking to the Guardian , said he made the movie "for the victims".
The same interview raises the issue of glamorising murder and serial killers - which Efron insists the movie doesn't do.
"Not glamorous or standing for a cause. Ted's dead. And karma... I wanted to make this film for the victims.
"Ted Bundy himself said that murderers don't come crawling out of the dark."
Collins is a busy bee at the moment, she also stars in biopic Tolkien.
She already auditioned for The Hobbit (2012) as an Elvin character but didn't get the part.
She is now playing Tolkien's wife who inspired the very character she auditioned for.
"I did audition for the Elvin character for the hobbit, I didn't get it. I became the woman who inspired them," she said.
Collins, who stars alongside Nicholas Hoult in the movie, was joined by Tolkien's great grandson Callum for the battle sequences.
Extremely Wicked, Shockingly Evil and Vile is on Netflix in the USA on May 3, and Sky Cinemas on May 3 in the UK.
