'Special Forces' Mark 'Billy' Billingham Recalls Being Angelina Jolie and Brad Pitt's Bodyguard (Exclusive)

ET has the first look at 'Special Forces: World's Toughest Test' that pushes Kate Gosselin, Jamie Lynn Spears and more to their limits.

Special Forces star Mark "Billy" Billingham is gearing up to test celebs to their limits. In the new Fox series, Special Forces: World's Toughest TestJamie Lynn SpearsKate GosselinMel B and more celebs will be put through 10 days of rigorous military training, which will include physical, mental and emotional challenges. The goal? To survive all 10 days of Billingham's bootcamp without quitting.

ET's Will Marfuggi spoke with the former special forces soldier about turning the demanding training course into a show and using his tactical training in the SAS to serve as a bodyguard to a few other famous faces, including Tom Cruise, Russell Crowe, Jude Law, Angelina Jolie and Brad Pitt, and more.

"I'll be very honest with you, people look at that job and think, 'Wow, it's glamorous, you're with these stars.' It's not like that at all," Billingham revealed. "It's the timelines you have to work. When these people do work really, really hard, when they go to bed after 14, 15 hours of work, I've still got three hours on top of that, so, it's the hours that you put in is really difficult, the sacrifice, 'cause you're away from your own people, your own family."

He continued, "These jobs as a bodyguard, especially at this level -- you know, everything about them, they know everything about you, so they won’t allow anyone to get close to them, and rightly so, if you bear that in mind, they're working all the time, so they expect you to be working all the time."

Billingham, who worked as a bodyguard for Jolie and Pitt for nearly two years, said it was a "wonderful" job, but a tough one, adding that you rarely got any time off while protecting the former couple.

"You never really get a break," Billingham admitted. "In the two years that I worked with Brad and Angie, or just less, very rarely did I get any time off. It was wonderful, I learned a lot of great things from them, like I hoped they learned a lot of stuff from me, but it's not a glamorous lifestyle. It really is a full on -- because you look over and you think, well, all my tenure of being a bodyguard, for every one of them, and call it whatever you wanna call it, but I never had an issue, or they never had an issue."

"And they often said to me, 'Well, Billy, why do I need you? Nothing really happens,'" he continued. "Because if I sit you down, and tell you why nothing really happens, it's because I'm doing my job. There are things going on that you don't see, and you don't need to see, and you don't need to know about, so, it's hard work. It's not glamorous, it's long hours and it's a big sacrifice, but it's worth it."

When it comes to training celebs on Special Forces: World's Toughest Test, Billingham goes even harder, stripping them back to their raw self to see what they can achieve under his intense training program.

"It’s all based on the special forces, particularly the SAS -- it’s the most physical, mental-demanding course I’ll ever do, 'cause basically SAS selection, special forces selection, believe it or not, is not about being a solider, it’s about, who are you?" Billingham explained. "And we're stripping, not breaking them. We don’t break people; we strip them back to their raw self. That means getting right inside their head, their emotions, how they think and how they tick -- what makes them, what they don’t like, what they do like, and then physically, we push them through boundaries they don’t believe that they can do."

While it's a show to the viewers, Billingham said he calls it a course for the celebs that participate, because that's exactly how he runs it.

"We take total control of their life, we even take their name away from them," he shared. "We give them a number, and they’ll answer to every single thing. It’s like being a prisoner to a degree, literally. We literally just take total control of them, and it’s a big culture shock to them."

It's also not the break from reality these celebs expect -- in fact, it's the opposite.

"They expect a break, producer intervention, we run the course, no one else, and they answer to us on absolutely everything," Billingham added. "They get shouted at, they get called things they can’t believe, and they expect a break, no. It's a thousand miles an hour, and they're expected to stay on it."

Calling the course brutal, Billingham assured ET that no celebrities were harmed in the process, though he did reveal that some had to leave the show for medical reasons.

"The SF world is based on the unknown -- expect the unexpected," he said. "There’s an old saying and I will say it, 'Be comfortable with being uncomfortable,' because I'll tell you what, that’s what we're gonna be."

Special Forces: World's Toughest Test premieres Jan. 4 on Fox.