Mark Billingham served 20 years in the elite fighting force, the SAS.

The star of Channel 4's gruelling programme SAS: Who Dares Wins has also been a bodyguard to Hollywood stars including Angelina Jolie, Brad Pitt, Tom Cruise and Michael Caine.

Speaking to Scotland Tonight he said: "It's only in the cold light of day that you come back from a job, and you're having a beer and a chat about it and think how the hell did I just walk out of that? It doesn't make sense".

Here is an edited transcript of the interview.

Mark 'Billy' Billingham: There were two reasons for going to the SAS, the first was, can I do it? I wanted to prove to myself I could do it. And secondly a lot of my friends before that had gone there, and I knew that they were doing amazing things although they weren't talking about it, so I thought I just want to do it. It's like becoming a footballer you want to go to the Premier, and the SAS is without a doubt the lead in the world in special forces. So I thought I want to be part of that.

Rona: And what kind of.. It must take a certain type of person, a type of personality, to make it, and endure for the amount of time you did in the SAS.

Mark: Well yes, I'm used to hardships, you know, I came from a poor background and I was used to fending for myself to a degree. The regiment and the military in general is about that. So I suited the role straightaway. I thrived on it, I loved it. I'm one of these people who just thrives in chaos, and produces my best result in a crisis. And being in the regiment I couldn't wish for a better job to be honest.

Rona: Looking back at your time in the SAS, do you ever think it's incredible that you have actually survived it? At the time are you just so in the moment and living on adrenaline, that you don't think about the danger?

Mark: You take it all for granted. It's only in the cold light of day that you come back from a job, and you're having a beer and a chat about it and think how the hell did I just walk out of that, it doesn't make sense. The cat of nine lives, I must be a double-cat.. Honestly, sixteen, it's just crazy. Everything from being almost shot at point blank range, to almost blown up and the bomb hasn't even gone off. Or it's been soaked up by something else. I've just been very very fortunate. A lot of luck.

Rona: And do you feel a bit invincible?

Mark: No, I just feel lucky. Not invincible. I just feel like maybe there's a reason why I didn't die that day, and I'd never have the answer to it, but no I don't I feel invincible. I just feel fortunate and lucky.

Rona: You then went on to become an instructor with SAS: Who Dares Wins, on Channel 4. Are you surprised that people want to come and do this?

Mark: People seem to have lots of different motivations to come and do this, really really gruelling programme. I'm not surprised at all to be honest, people read the books and want to know more about it, and if they get a chance without doing the whole thing and being committed to it, then they'll want to do it. And there are thousands of people who apply. A lot of them want to prove a number of things to themselves, one - they can take a physical show challenge, they can prove good things to themselves. And a lot of them seem to have something that they want to get off of their chests. A lot of it is quite emotional, and some of it is disturbing. But they all walk away feeling better, and having, you know, shared a problem, and hopefully we've helped to give them some sort of solution to the way forward. And it seems to be working.

Rona: What you do fascinates everyone, and when you go on a speaking tour what are the questions that people ask?

Mark: Everyone wants to know about your life in the SAS, your operational side of it. And unfortunately I can't talk about a lot of it. But they just want to know what drove me to be there, and generally what it was like. That is the main question. You always get the odd question about how many lives have you taken as opposed to how many lives have you saved, you never get asked that question. But generally most people are very respectful and they enjoy the talk, and they want to know what ingredients did I have to get from where I was, a naughty schoolboy kid from the council estate, to being in the finest regiment in the world. And not just being in it but being one of the leaders doing amazing jobs. They're the sorts of things that people want to know.

Rona: Part of your career also has been an A-list bodyguard to people like Angelina Jolie, Brad Pitt, Tom Cruise, what was it like going from the battlefield to what appears to some as a really shallow world?

Mark:Luckily I'm not a starstruck sort of person, they were more in awe of me that I was of them for sure. And what I really loved with everyone I worked with was respect. They totally respected what I did and what I know. And if I told Brad, Angie, Michael Caine, whoever it was, this is not happening, for these reasons. They never questioned it. Which I found was really respectful, but I was always mindful about what they do as well, so it was a mutual understanding. And it was a fantastic time, I learned a lot from most of these people. What people don't see is how hard these people work. Even when they are not on camera they are doing charity stuff, they spent time with their kids, they are fantastic time managers. And I learned a lot for them. It was an awesome time.

Rona: I suppose the question here is, when you are taken on as their bodyguard, use you supposed to take a bullet for them, how far do you go in protecting their lives?

Mark: I tell you what a good bodyguard does, is not take a bullet for anyone. A good bodyguard is not sculptured, polystyrene, 6 foot 6, big muscles, it's actually what's in there, it's thinking about it. And that's 90% of your work, understanding what the threat is, and how you can get round it, without scraping.. and being caught up in something. If you're my client and someone is shooting at you, I've got you in the wrong place. You know, someone's trying to attack you, and I'm rolling around on the floor, then I can't protect you. It's about thinking and avoiding it, that's what bodyguarding really is, you're on your toes all the time.