The native Canadian maintained her acting career until her death, and recent credits include Rob Zombie's "Halloween II" in 2009, as well as a series of Canadian films, including "The Neighborhood" and "The Red Maple Leaf."
D'Angelo said Kidder had to miss taking a role in his 2015 movie "Sicilian Vampire" because she got sick protesting the Keystone Pipeline.
"I heard she was supposed to go to a convention but said she had been fighting the flu. The flu and pneumonia can be detrimental at any age," D'Angelo said. "She was a fighter, so something had to hit her hard for her to pass so suddenly."
"The irony was that security wouldn't let her back on. They thought she was an actual homeless person. She didn't get mad. She laughed about it. She was a great lady," he said.
"She told me she was fighting the pipeline and got sick sleeping outside," he said. "She was one of those ladies who would fight for what she believed in."
"I can't say anything more, I'm too upset," Perrine said.
Stringer said Kidder loved animals and advocated for their use in the treatment of people living with mental health disorders.
Outside of Krypton, Kidder maintained a steady stream of roles, including the 1981 film "Heartaches" and a 2002 Broadway gig in "The Vagina Monologues," as well as appearances on TV shows in the 2000s, including "Law & Order: SVU," "The L Word," "Brothers & Sisters," "Smallville" and "R.L. Stine's The Haunting Hour," which won her a Daytime Emmy in 2015.
Kidder and Reeve reprised their roles as the famous duo three more times, in 1980's "Superman II," 1983's "Superman III," and 1987's "Superman IV: The Quest for Peace."
"She wanted to talk a lot about mental health issues and how animals can help people who are suffering," she said.
Kidder said that once she was taken by her siblings to the hospital, and introduced to the writing of Kay Jamison, she was finally able to accept the manic-depressive diagnosis she'd received in 1988.
Kidder's career was sidelined, however, in the mid-1990s after a manic episode of her bipolar disorder made headlines.
Margot Kidder, who starred as the Lois Lane to Christopher Reeve's Clark Kent in four "Superman" films in the late '70s and '80s, died Sunday at her home in Montana, her manager confirmed. She was 69.
After turning her life around, Kidder reflected on the tumultuous decade in 2005, saying she was happy to have finally come to terms with her demons.
"She was Superman's girlfriend. They had undeniable chemistry. It was there," she said of the onscreen couple.
News of her death prompted a flood of tributes on social media, including messages from Mark Hamill and Maureen McCormick.
Before landing the gig that would go on to define her career, Kidder appeared in the 1973 cult thriller "Sisters" as Siamese twins, as well as two horror films — "Black Christmas" in 1974 and "The Amityville Horror" in 1979, as one half of the tortured married Lutz couple.
"She was definitely an important part of the fabric of our community," Stringer said. "She was very involved in lots of political campaigns. She supported Bernie. She did a lot to open an office in our local area to get Barack Obama elected."
He recalled Kidder played a homeless woman in his 2017 Danny Aiello flick "The Neighborhood" and had to leave the set at one point in her costume.
The film shot her to international fame — something she later called "the weirdest thing in the world."
Kidder is survived by her daughter, Maggie, and two grandchildren.
"I still get stopped for being Lois Lane, and I'm 60 and have two grandchildren," she told The AV Club. "So it's kind of weird."
She was found days later in a Los Angeles backyard with her front teeth Krazy Glued in place after they fell out during an alleged rape attempt.
He said Kidder may have recently been battling the flu.
The actress disappeared for four days in 1996 after the computer on which she was writing her autobiography crashed, deleting three years' worth of work and sending Kidder spiraling.
"She was just awesome," he said in a phone interview. "She was great on the set. Everybody loved her."
"My boyfriend saw her two weeks ago petting somebody's puppy. She was hanging out with someone and their dog," Stringer, who also lives in Livingston, Mont., said.
She scored the coveted role of Superman's paramour Lane in 1976 after a lucky call earned her a screen test in front of director Richard Donner.
"On-screen she was magic. Off-screen she was one of the kindest, sweetest, most caring woman I've ever known. I'll miss you #MargoKidder. Your legacy will live on forever," Hamill wrote on Twitter.
Kidder, who reprised her role as Lane in three "Superman" sequels in addition to the 1978 original, was a longtime mental health advocate, as her battle with bipolar disorder played out in the public eye in the late '90s.
Police do not suspect foul play, though the actress's death is under investigation, as authorities found her dead after responding to a call from an unknown person who claimed she was unconscious and not breathing, Park County Attorney Bruce Becker told the Daily News.
"Horrifying as it was to crack up in the public eye, it made me look at myself and fix it," she told The Guardian. "People were exploitative, that's human nature.. But you take the cards you're dealt, and I got better. I'm now ferociously healthy in body and mind."
"That's when I went from really distressed to absolute delusion," she explained to People in 1996, adding that she started to believe McGuane and the CIA were plotting to kill her.
Perrine played Lex Luthor's girlfriend and accomplice Eve Teschmacher in both "Superman" and "Superman II."
Meanwhile, former costar Valerie Perrine, 74, expressed shock at the loss of her friend.
She said Kidder's legacy is clear.
In addition to her marriage to McGuane, Kidder was briefly married to actor John Heard and French film director Philippe de Broca. She also famously dated former Canadian prime minister Pierre Trudeau and comedian Richard Pryor.
"I'd never read comics, so I didn't know much about Superman," she said in 2009. "But I read this very funny script, and I went in and did a couple scenes, and next thing I knew, I was being flown to England to screen-test, and that was that."
"Margot was an icon. This is all very shocking. No one saw this coming. My understanding is she went to sleep and didn't wake up," Kidder's friend and former assistant Janet Stringer, 48, told The News.
"I can't believe she's gone. She was always happy and smiling," Perrine told the News of Kidder. "It was so easy working with her. She hit her marks. She knew her dialogue. And she was kind. She was my friend."
Kidder has said that signing on to the film also marked the end of her first marriage to novelist Thomas McGuane, the father of her only child, daughter Maggie, as he wanted her to be a "subservient writer's wife."
Canadian filmmaker Frank D'Angelo did five movies with Kidder in the last decade and recently spoke to her by phone about a new script he was writing, which also included a part for her, he told The Daily News on Monday.