Barnett: Real fires lit on set
The actor, who plays fireman Peter Mills in the US drama, admitted that the explosions are not computer-generated, which lend an air of realism to his acting.
"I know when I'm running into most of those buildings it's hot and there's a real fire in front of me - it's not CGI," he said.
"All the tools are real and that's largely down to (executive producer) Dick Wolf; he made sure we had everything possible to make it real - the tools, the trucks, the gear, all of the people in the background are real firefighters."
He added: "If I have to use a Halligan (firefighting instrument) in a weird way or it's written strangely and I haven't spoken to the writers then I've got a firefighter there to explain it and talk about the experiences he lived through. It makes it so, so real for me, especially being from theatre."
Charlie revealed that he can relate to Peter easily.
"Peter fits me very well. We're very similar and in any kind of TV show the further you go on the more parts of yourself begin to bleed into the character; the two fuse together which is really interesting," he explained.
"What drew me in was the blend of excitement, family and intimacy as well as the fact that anything could happen at any point - it's risky."
But the role has its challenges. He added: "Having to figure out how he fits into the group and having to figure out how I fit within this group of actors, crew and producers, it's scary sometimes."
:: Chicago Fire Season One is out on Blu-ray and DVD now, from Universal Pictures (UK).