But for two years, that's just what Mithaq was.

Beardless and with a light, hesitant voice, the 32-year-old says of the terror group he once served: "I liked them, so I wanted to join.

"I was motivated by my love of Islam," he adds, handcuffs jingling on his wrists.

When Australians think of IS fighters, notorious foreign exports come to mind: recruiter Neil Prakash, wild-eyed Khaled Sharrouf, or young Jake Bilardi, who blew himself up for nothing.

But many of this movement's footsoldiers are ordinary Iraqi Sunni Muslims, often from rural areas and with little education. Frustrated and feeling abandoned by the Shiite-dominated national government in Baghdad, they were swept up in a common cause, believing in IS they had found a better, purer future.

Mithaq was such a man. A farmer with only a sixth-grade education, he says of the so-called Islamic State: "I thought they were the real Muslim caliphate."

Now, under fluorescent lights in a bleak concrete room in a police base in the northern Iraqi city of Kirkuk, Mithaq has plenty of time to regret his delusion.

His farm, which he says once provided well for his family  - more than the IS wage of $250 per month - lies abandoned. His two children, aged three and four, are without their father. His wife long ago told him she disagreed with his choice.

Mithaq's interrogation will establish his real role in the militant group. Depending on what Kirkuk's anti-terror police find, the sentence could range from five years' imprisonment to the death penalty.

So Mithaq has a strong interest in downplaying his involvement.

"I was a guard at checkpoints," he says, "Nothing else."

When pushed, he adds: "I joined some battles. I never killed anyone directly, but I did join some [military] operations."

When he first joined IS in mid-2014, life was "very good". There was food, services, a common religious cause. His wish for a pure Sunni state seemed to be coming true.

He underwent military training which - depending on the specific skills the recruits were learning - ranged from 20 days' duration to two months. Each day included six hours of training and one hour of religious indoctrination.

After several months, though, an order came from the emirs - the religious-military  commanders above him - and everything changed.

Punishments were stepped up. Not having a beard was a punishable offence for a man. They couldn't wear their trousers too long, nor women wear their abaya [robe] too short.

"I don't know why," he says, "I wasn't an emir" - and they couldn't be questioned.

Smoking, mobile phones, contact with the outside world, nail polish and high heels were all banned. Running away was a capital offence.

"If someone had done something wrong, you could just punish them by cutting their head off, or their hand, or any part of their body."

He was present when people accused of stealing had their hand placed over a wooden table and cut off with a knife.

Mithaq sighs: "I was getting sad when I saw these things."

The point was, he says, to induce fear: "That, if someone does something wrong according to [IS] rules, that this will be his fate".

Even those who had joined IS were fearful. Those who abandoned their post or communicated with the other side would be executed in public, or taken away, shot, and dumped somewhere, unburied.

At Mithaq's checkpoint, fear made for shallow friendships and boring conversations.

"We would talk always about Islam, deen [faith] and jihad [just war]. That was the only thing we would talk about."

In their free time?

"Still ... we would talk about fighting, military things, operations, guns. It was forbidden to say other things, to talk about other things."

Hypocrisy, though, flourished.

"Even the emirs used to smoke. I didn't see it for myself, but I heard … that there were some emirs smoking … They could not give up," Mithaq says.

"I don't know why. We couldn't ask our emirs if they were smoking or not, or punish them … [Also], the emirs would take [ethnic minority] Yazidi girls without getting married to them."

Starting late last year, the so-called Islamic State was pushed back under military pressure from Iraq, assisted by Australia, among other countries.

The emirs from the Kirkuk region abandoned their men. As usual, there was no explanation, but Mithaq believes they fled north, to the remaining IS stronghold, the city of Mosul.

Mithaq wasted no time.

"I came to Kirkuk [city] because I ran away from Islamic State. There was no food left and they were criminals."

"He sneaked into Kirkuk with civilians" according to his jailer, Colonel Fattah, who prefers to use only one name. Mithaq was caught when his name matched a list compiled from information gleaned by interrogating other captured fighters.

Brigadier-General Sarhad Qadir Muhammad in his office. The flag of Iraq's Kurdistan region can be seen on the right. Iraq's government has in the past disputed Kurdish authority over Kirkuk province. Photo: Kate Geraghty

Police Brigadier-General Sarhad Qadir Muhammad, head of the counter-terrorism division of Kirkuk province and a former Kurdish guerrilla, says 340 former "militants" have been arrested in his area in the past year. More than 10 men had tried to escape detection dressed as women in the all-covering female clothing demanded by IS – an action he regards as cowardly.

As a relatively low-level player, Mithaq's sentence is likely to be 15 years in jail, Colonel Fattah guesses. But the Iraqi state  administers the death penalty by hanging.

As the subject is discussed in front of him, Mithaq slumps in his seat. His head dips between his knees; he wrings his cuffed hands and begins to breathe in gulps.

After the false promise of Islamic State, the reality is hitting home.