IT WAS the smell that hit him first. The sort of smell that is so strong it hits you like a punch to the stomach and almost brings you to your knees. It clung to his skin for days afterwards, a reminder of the horror he had just witnessed.
He may play Jimmy Palmer, assistant to NCIS medical examiner Ducky, where his day job requires him to come face to face with all manner of horrible deaths.
And as an actor, he has encountered "dead'' bodies on set more often than many people have tied their shoelaces, but Brian Dietzen admits sitting in on a real autopsy is nothing like doing one on set.
"It was awful, just awful," he said of the first time he sat in on one in LA as research for the role.
"I took a little notebook and kept my head buried in that. I didn't want to look at what they were doing. It was really gruesome and quite confronting to see bodies being treated that way, simply as pieces of evidence.
"I'd only ever seen a dead body at funerals where they are treated with the utmost respect and reverence. To see the mid-process was intense."
He's played Jimmy Palmer for all 10 seasons of NCIS and each year attends an autopsy to refresh his knowledge and to ensure his character is equipped with the latest developments and technology. He's pleased to admit he hasn't become desensitized to the experience.
Despite having been on the show since the beginning, it was only at the start of its tenth season that he was made a series regular.
"People keep asking if I'm upset it took so long, but I believe things happen as they are supposed to. If you work hard, good things happen.