Their targets during such a statement are simple - they have to make a variety of noises of incredulity,using any raw material that comes to hand, from disbelieving astonishment to caustic levity.
The responsibilities of the shop-floor workers on the other side are equally straightforward. They must produce a rhubarb of gratified surprise as each measure is unveiled - a cross between the orphanage Christmas party and a particularly stimulating trapeze act.
But while Labour MPs more than met their quotas during Gordon Brown's performance (despite a grumpy go-slow by the Welfare Bill rebels) the Tories' heckle production fell steadily throughout his speech. By the time the Chancellor had finished, they brought to mind the gloomy indolence that grips a Siberian fridge factory as the first snowflake falls on the tundra.
It wasn't too bad at first - since Mr Brown chose to begin with a parade of vintage fiscal cliches from the Treasury yearbook. Last year he promised to steer a course of stability, he reminded us, and this year he proposed to set a course for the same destination.
Tories jeered happily as each old friend drove by, and roared with nostalgic pleasure as "boom and bust" brought up the rear. And though Labour MPs shouted at them to "cheer up" as Mr Brown issued his new growth forecasts (all up on last year), the yells were still just gamesmanship at this point. It was too early for genuine depression to have set in.
True, there was something ominous about the way the Chancellor was using a half-yearly update to issue promises datemarked for 2010. Even if nobody else believed his figures, it was clear Mr Brown did, and was beginning to think not just of the next term as an inalienable Labour right, but the one after that as well. But there was plenty of time yet for him to go wrong.
When Mr Brown moved on to his policies to encourage enterprise, the Tory silence took on a new quality: capital gains tax would fall on long-term investments, the Chancellor announced with a brutal smirk, and with every percentage drop he detailed, Tory faces fell a little, too. Some looked as if they were trying to remember where they'd left the keys to the gun.
As Mr Brown outlined the new measures on employee share ownership and competition policy the faces slumped even lower - a mighty factory of scorn fallen silent.
There were brief rallies. When Mr Brown announced that benefit claimants might have to sign on every day, rather than every two weeks, several MPs roused themselves to shout "Thief!", recognising that this particular item had been nicked from the Conservative "common sense revolution".
And when the Chancellor announced that the fuel taxes might not ascend at quite the same rate in future, there was another surge of indignation from Tories - for months they'd been encouraging him to run down the "up" escalator and now the killjoy proposed to hit the emergency stop button and deprive them of one of their few remaining parliamentary pleasures.
Mr Brown likes to toy with his prey before he kills it and he had kept his most flagrant headline grabber till last.
Some people had suggested that pensioners over 75 should be assisted by a reduction in their television licence. "I have rejected that option..." said Mr Brown sternly. A five-year-old could have seen what was coming next - free TV licences for 3 million pensioners.
Mr Maude, foreman for the proceedings, made a brave attempt to get his men back to work, but most of them seemed to have given up the day as lost.Reuse content