'It's the Tarantino moment: we are all spattered by blood'

Neil Pearson watched a live operation to make Monroe more realistic

Medical drama: Neil Pearson and Sarah Parish in 'Monroe'

I'm in an operating theatre in Wythenshawe Hospital. Soon 70-year-old Malcolm will arrive on a trolley, and a thoracoabdominal aortic aneurysm will be removed from his mid-section. Open-heart surgery aside, the removal of an aortic aneurysm is the most invasive planned procedure a surgeon performs. I'm going to watch.

Usually there are two ways to get into an operating theatre: train for seven years, or be very, very ill. A third way is to hitch a ride as an actor. The medical consultants on Monroe have made some calls, and I will now observe as Dr Mo Baguneid slits Malcolm from breastbone to pelvis, and unclogs him.

A small man with a big presence, Mo has the kind of quiet, amiable confidence you like to see in a man whose job involves cutting you open. I'm talked through the operation, sparing no detail. A squeamishness test. I pass. We head to theatre.

Malcolm is on the operating table, unconscious, anaesthetised by an epidural and chemically paralysed from the waist down to prevent any involuntary convulsions. Anaesthetists conclaved at his head, his body is covered save for a rectangle of flesh running from the bottom of his ribcage to his pelvis. I met Malcolm briefly before he was taken into theatre to wish him all the best. I'm about to see more of him than I've ever seen of anyone in my life.

The aorta is the body's primary blood supply, sprouting from the top of the heart before doubling back down to just below the navel, where it forks to carry blood to both legs. An aneurysm is a blockage of the pipe, reducing bloodflow from a fast-flowing river to that of a pram-filled canal. Form a circle with your thumb and third finger: that is the diameter of a healthy aorta. Malcolm's aneurysm is ten centimetres across. If an aneurysm bursts, you die.

Mo makes the first incision, travelling from the sternum, around the navel, and down as far as the pubis. A laser burns through the fat, the incision becoming a wound; spreaders turn the wound into a cavity. In here, among everything keeping Malcolm alive, is something that is threatening to kill him.

Mo rearranges the intestinal furniture, taking a couple of handfuls of Malcolm's viscera and balancing them daintily on his patient's hip bone. Their displacement uncovers an organ I don't recognise. Black, hefty and throbbing, it looks like a Christmas pudding with a pulse. The aneurysm.

Clamps in place, the aneurysm is slit open. It's the Tarantino moment: thick, black blood splatters all four people around the wound. Mo stretches the incision and scoops out a lifetime's detritus: a gelid, blackened mess which fills a large kidney dish. Then he calls for a spoon, hospital parlance for... well ... a spoon, and uses it to scrape the aortic shell clean of gunk as if he was polishing off one of those little cheesecakes that comes in its own glass pot.

From here, the operation is time-trial embroidery. Ducting is laid through the defunct section of aorta, ends are sewn, seals are checked, blood supply is restored. No complications. It's been beautiful to watch – not an adjective I expected to use today. I'm rather proud of my species.

A couple of weeks later I visited Malcolm in hospital. He was grumpy, homesick and uncommunicative. Fine by me: Malcolm had had his Malcolmness restored to him, and that was what the operation was for.

A few weeks later we filmed the aneurysm sequence for Monroe. Mo had taken four hours. We took six.

'Monroe' continues on Mondays at 9pm on ITV1