In Israel Mr Shearer ran the UN office based in Jerusalem coordinating assistance to Palestinians in the West Bank and Gaza.
He said he was always conscious of the tension between Palestinians and Israeli security.
"It wasn't uncommon to see an Israeli soldier hassle an old Palestinian man. The young soldier with his boots and his weapon, and the old man frail and hunched, clutching whatever it was he was bringing home from the market."
Mr Shearer said he learned to look at situations from both perspectives to learn that often Israeli soldiers would see Palestinians as the enemy and were simply products of the system.
He said negotiations were mainly about security checkpoints - negotiating numbers, opening hours and neighbourhoods.
"Success for me meant more Palestinians got to hospital, school, markets or to their jobs."
Mr Shearer told those gathered he had a standoff with an Israeli army colonel who insisted on bringing his gun into the office which had a strict no guns policy.
"I always refused. No guns."
"One day Tony Blair was coming in - he said he wouldn't get Blair to our office if his weapons were banned."
"Blair did visit and there weren't weapons," he said.
The Labour leader told an audience about being driven to a camp in Liberia by a Dutch man just after he'd heard there was cannibalism in the area.
"I asked him: 'is there anything to what they're telling me about the cannibalism?"
The man told him it was a huge exaggeration.
"He went on to say - 'All they do is, after killing, quickly open the victim's chest, and eat the still beating heart."
"He then said, 'that sounds a lot like cannibalism, I guess'."
Mr Shearer said the work could leave you a little jaded and hardened but there were many successes that had fuelled his political work.