The Hobbit star plays neurosurgeon Gabriel Monroe in the British show and in a bid to make his portrayal as realistic as possible, he attended up to five skull operations at a hospital in Leeds, England.
The actor admits he was apprehensive about watching the gory procedure, but once he had overcome his nerves he found the whole process fascinating.
Nesbitt tells U.K. TV show This Morning, "We were very fortunate in that I got to spend a lot of time with surgeons. I sat in on four or five brain operations. The first was extremely difficult. For the first one I was just stood at the back. I didn't know what was going on.
"To get into the brain is very agricultural and industrial - they're kind of drilling and sawing. Then the surgeon called me over. Once the brain is revealed it's a rather beautiful and stunning kind of organ... And he said to me, 'See that bit there? That's thought'."