While Christopher Reeve was known to most people as Superman, to his three children – Matthew, Alexandra and Will Reeve – he was simply their beloved father.

The three siblings witnessed firsthand how their father evolved from movie star to pioneer of spinal cord research after a near-fatal horse riding accident left him paralyzed from the neck down at the age of 42 in 1995.

Twenty years after Christopher Reeve’s death from heart failure in 2004, his life, work and legacy are examined in the new documentary “Super/Man: The Christopher Reeve Story,” which also features all three of his children.

The Reeve siblings sat down with ABC News’ Diane Sawyer to talk about their memories of their father and his wife, Dana Reeve, who died of lung cancer in 2006, just two years after her husband’s death.

“I believe that no matter what a family looks like, no matter how shaggy the branches of the family tree are, as long as they are rooted in love, it doesn’t matter what your family looks like, as long as it is your own,” Will Reeve said of his family’s perseverance in the face of tragedy.

Will Reeve, now a correspondent for ABC News, was just two years old when his father had his riding accident. His half-siblings Alexandra Reeve and Matthew Reeve were 11 and 15 years old respectively.

The siblings remember the first moments when they saw their father again after the accident.

“Dana led us downstairs and said, ‘It’s going to look really scary, but he’s still there. Just talk to him. Ignore everything else. Just talk to him,'” Alexandra Reeve recalled, adding, “She said, ‘You can hold his hand.’ And there are machines on it, you know, and you grab his fingers and hold them.”

Alexandra Reeve, now a lawyer, remembers holding her father’s hand amidst all the machines working to keep him alive.

Matthew Reeve recalled that when he saw his father for the first time after the accident, he simply said, “I love you. We are here for you.”

“I think to make sure I wasn’t permanently afraid of my dad, my mom (Dana Reeve) made sure I was involved as much as was appropriate,” Will Reeve added. “And that included always being near him, touching him, helping him lift his legs up to stretch him, which is important for someone with a spinal cord injury.”

When Christopher Reeve was released from the hospital, they learned from observing Christopher and Dana Reeve that life goes on and that the love in the family, although different, remains the same.

Will Reeve said his memories are of a “happy” and “noisy” home full of family meals and joy.

“Whether it was good news, bad news or scary news, dinner was family time,” said Will Reeve. “My mom cooked (and mom and dad) sat at the head of the bed, next to each other. My mom fed him and herself.”

He continued: “We had friends come over. It was very happy, boisterous, loud – everything you want from a family meal, every night. And the only thing you weren’t allowed to talk about was specific medical things. It could have been anything.”

Matthew Reeve recalled a house “full of music” thanks to Dana Reeve, who was a professional singer when she met her future husband at age 25.

“She would always just start singing, and that’s definitely one of the things I miss the most,” said Matthew Reeve.

Will Reeve also recalled that his mother was “always singing,” adding: “You’re always wiser in hindsight, but I wish I had asked her to sing more because when I think of her, it’s those quiet moments of safety and happiness and normality and being together. We didn’t even have to be in the same room, but I knew where she was. I miss that.”

Like other children, the siblings looked for ways to make their father laugh throughout their childhood.

“Matthew brought a gift for his dad at some point in college. It was an eject button that he could attach to the hand of his wheelchair, right next to his finger. Dad loved it,” Alexandra Reeve recalls. “Someone is in a serious meeting with Christopher Reeve and suddenly he sees his wheelchair. He just sees this eject button and you can see it on his face, like he’s asking, ‘Ask? Don’t ask? What should we do?'”

In the decade following his accident, Christopher Reeve inspired not only his family but millions of people around the world as an activist and advocate for spinal cord injury research. He founded the Christopher & Dana Reeve Foundation with his wife.

According to the foundation’s website, it has since invested more than $140 million in research.

Remembering the legacy of Christopher and Dana Reeve

Less than a year after delivering the eulogy at her husband’s funeral, Dana Reeve, a lifelong non-smoker, was diagnosed with stage four lung cancer.

She died seven months later, on March 6, 2006, at the age of 44.

In the documentary, Will Reeve said that it was only after his mother’s death that he read her diaries and fully understood the difficulties she faced as a caregiver for her husband and family.

“I have studied the difference between loneliness and being alone. I tell my life story to the clean white towels that I take warm from the dryer and press to my chest. A sad substitute for a body that I hold close to me. I think of him in certain lighting conditions. Dawn, late afternoon, bright, windy days that would be perfect for sailing,” Will Reeve reads from his mother’s diary in “Super/Man”. “Even now, what I miss most are his hands. Their expressive grace and weight. The warmth of his hands on my skin. The embrace of his arms, two becoming one. I carry the stack of towels upstairs and hold them carefully so they don’t fall. Except for one that is still damp, the top one that I had pressed against my face and that still needs more time to dry.”

Super/Man: The Christopher Reeve Story premiered at the Sundance Film Festival in January. The film is scheduled to be shown in select U.S. theaters on September 21 and 25. Tickets are available through Fathom Events.

ABC News’ Carson Blackwelder contributed to this report.