Rather like the mechanism that induces nervous hilarity when least appropriate, it takes very little to trigger laughter in court.
Such is the tension around Court No 2 - where sombre matters of life and death, loss and grief, loyalty and betrayal are the stuff of everyday discourse - the appearance of a combative bill-poster yesterday was enough to bring smiles where few have been seen for three weeks.
The first and only non-Garda witness of the day, Wesley Kearns shuffled in, head down, looking like a man being assailed by wasps.
A former employee of Viacom, the outdoor advertising company where Joe O'Reilly worked as an operations manager, Kearns had worked there for nearly two years when O'Reilly came along and was hardly in the place a month before he started throwing his weight around, according to the witness.
There was disagreement over whether a poster had been put up. Kearns argued that he had put it up; O'Reilly gave out to him and said he had not.
The poster was up later that day but things then clearly took a further turn for the worse. The accused man fired him, whereupon Mr Kearns hurled a fire-extinguisher at him, causing Mr O'Reilly to flee for his office, locking the door behind him.
Kearns's rapt audience in Court 2 broke down in titters.
"Did you like him?" asked the State prosecutor, Denis Vaughan Buckley. "Yeah, I did like him. Yeah," said Mr Kearns, apparently bemused at the question, and pausing a beat, "up till the day he sacked me", upon which he made the considerable effort to turn right around to face his nemesis. The court - united for once in a single emotion - dissolved into laughter.
There was a point to the questioning. Asked what he was doing on October 4th, 2004, the day of Rachel O'Reilly's death, he said he was at the labour exchange in Clondalkin between 9am and midday.
But he had been re-employed by sub-contractors to Viacom, suggested defence counsel Anne Rowland in her soothing voice.
And Mr O'Reilly wasn't happy about him rejoining the bill-posters? "Didn't like it. Didn't like it," averred Mr Kearns, noting that he had been barred from one side of the factory. "But he didn't stop you," said Ms Rowland. "Didn't need to, didn't need to," said Mr Kearns stoutly.
Before Mr Justice Barry White sent the jury home for the weekend he reminded them to ignore media reports of the case. He had not read the day's papers himself, he said, but he had heard It Says in the Papers, which "referred to some 'lurid' headlines". Doubtless he was referring to Paddy Clancy's review of yesterday's papers on RTÉ's Morning Ireland, in which the reviewer noted that "every paper in the Republic gives page one prominence to [ the O'Reilly trial] bar The Irish Times. The evidence of O'Reilly's former mistress, Nikki Pelley, produces the material for most headlines, three tabloids being especially lurid".