'I witnessed my twin sister being attacked on the street, and I was paralysed.'

I remember in one of my first psychology lectures, we were told the story of Kitty Genovese.

Kitty lived in New York City, and was stabbed to death in front of her apartment building, while almost 40 people saw or heard and no one intervened.

The details of Kitty’s attack are often shared as a prime example of the ‘bystander effect‘, a social phenomenon whereby people don’t offer to help a victim when other individuals are present. Psychologists say this happens for several reasons, including uncertainty about the seriousness of the event, perceived differences between the victim and a potential helper, and diffusion of responsibility.

Despite my knowledge of the bystander effect, I always naively assumed that if I was ever in a crisis situation – terrified and in desperate need of help – people would come to my aid. They’d have to.

But then, on a typical Tuesday afternoon when I was 22, I found myself in a moment of crisis. And no one came to help.

My twin sister and I were walking down the street, in broad daylight (wearing frumpy tracksuits for those of you who will inevitably wonder in a minute) when we noticed a man walking towards us. We noticed in the same way you notice any small, insignificant details within your day. It's one of those things that would normally fade into obscurity, had it not been for what happened shortly after.

Because as he walked past, he violently grabbed my sister.

We had been walking side by side, chatting about our day, when he took my sister, and pushed her to the ground.

My heart stopped. In an odd way, I felt like this was something I never thought would happen, and something I always thought would happen. As a young woman, you're primed to believe that something horrible is going to happen to you or someone you know at the hands of a man who sees you as an object rather than a human being. And this was that moment.

My sister screamed and fought, as this man held her down and tried to get his hands under her clothes.

I screamed at him. I yelled for him to get off her and stop, but I couldn't move. No part of my body could move. I was completely frozen.

I continued to yell and scream and scan the nearby area for help. I could see at least two people - one at each end of the street. I stared at them. I yelled in their direction. I pleaded with them to help. But they just stood there, stunned.

I looked at my sister, helpless and small lying on the ground while this man had complete control. I didn't know if he had a knife, or a gun, or whether he was going to rape her, or beat her, or take her away. And despite feeling more terrified than I have in my entire life - I just couldn't move.

While I yelled at my sister's attacker, I began saying that I would call the police. The words came before the thought. But once I said it, I realised I had to do it. So shaking and angry and scared, I pulled my phone out and dialled 000. I continued to tell him I was on the phone to the police and that they'd be here soon.

Once I started speaking to someone on the other end of the phone, the man stood up. He looked at me with a maniacal expression, pulled down his pants, and started masturbating just inches from where my sister was lying on the ground. He then turned and ran down the street, as I scrambled over to my sister, still shaking, and starting to cry.

My sister has previously written about her experience, and as a result, we were asked to be on Insight, discussing first responses to crises.

The discussion seeks to explore what it's like to be first on the scene in an emergency situation. How do people react? What influences how we behave in a crisis? Is it a conscious choice, or are we acting on instinct?

The stories on the episode - of terror and bravery and perseverance and selflessness - truly take you to the core of what it means to be human.

Watch Peter Davidson reflect on his most chilling moment as a paramedic. Post continues after video...

Alastair Boast was sitting in a tutorial when a fellow student pulled out a gun, and began shooting.

Joel Trist was surfing when he heard a spine-tingling scream from his best friend. He had been attacked by a shark.

Peter Davidson was the first responder for the infamous 1998 Sydney to Hobart.

Lydia Johns-Putra was in Christchurch for a conference when an earthquake hit, and as a urologist, she had the unique skills to help.

As you'll see on Insight, crisis situations often bring out fortitude and unquestionable strength of character. These people behaved in ways many of us could never imagine. But according to researcher Rachael Sharman, we don't necessarily choose how we respond. And our memories of those crucial moments and our motivations within them are often hard to recall, because they were never processed in the first place.

After the show, Sharman assured me that in my situation, I had 'done the right thing.' I don't think she realised how much I needed to hear that. I've always felt weak and ashamed for not being able to move when my sister needed me most - and I think I always will.

But as I heard the stories of these people who have been heroes in emergency situations, I realised we all have one thing in common - regardless of our response.

We share the experience of having your understanding of the world shatter before your eyes. Of believing that you're safe and secure and nothing can hurt you, and having circumstances tear apart that illusion. We share the nightmares and panic and emotional toll that comes with being entirely terrified for a moment in time - and the change in perspective that comes when you realise you've been profoundly and irreparably changed by it.

No matter how you respond in a moment of crisis, whether you're a bystander or a hero, your world changes. As Rachael Sharman says, perhaps you can't choose how you respond to an emergency.

Perhaps we can't blame the people who stand in the street, while a young girl calls out for them to help. Perhaps I can't blame myself for freezing.

Perhaps we should view all responses to these situations as equal and as having their own value. Because no one asks to be put in a terrifying situation, and we don't always know the 'right' thing to do.

We're human - and nothing reminds us more of the fallibility and complexity of our condition than being in the throes of crisis.

You can watch Insight: First on the Scene via SBS On Demand.