Producers obviously want the best guests, but they're cruelly limited by the fact that said guests must a) be alive, and b) exist on the same plane of reality as the rest of us. That, of course, rules out the dead. And angels. And unicorns. All you're left with are living humans, and we all know how boring and irritating they can be.
Those tuning into Wednesday's Moncrieff probably didn't expect to hear these 'immutable' laws of radio (and science) shattered, but that's what happened.
Moncrieff's opening remark about how "everyone has an opinion on today's Budget" was but the set-up for a mind-boggling twist. He wasn't merely about to ask some tedious dial-a-pundit what they thought. He wanted to hear the views of "the founding fathers of our State". Eh?
"We asked medium Helen Feeley to have a go at contacting Padraig Pearse in the afterlife to see what he thinks," explained Sean (if you can call that an explanation).
"Did Padraig have plenty to say?", he asked. "He had loads to say!", replied Helen, chirpily. "We had a good chat last night... myself and Padraig." The Pearse she described was unquestionably an affable old soul, but one who seemed (disappointingly) to have gained little in the way of genuine insight in the 96 years he'd spent being dead.
Ireland's current woes were summed up as follows: "We just grew too big, too soon. And then the banks collapsed." Succinct, admittedly, but you'd expect more than triteness like that from creatures who dwell in a transcendent realm of pure spirit.
When Pearse wasn't offering banal comment about the present, he was casting his Nostradamus-like gaze into the nation's future. It wasn't exactly earth-shattering. He'd told Helen that "semi-states will become accountable and profitable" (Ooh!) and, somewhat worryingly, that volunteering "as a charitable thing" would become "compulsory".
There would also, she/he went on, be "a battle for leadership", resulting in "a labour leader... for the next seven to 10 years". You probably imagined the afterlife as a carefree place of endless bliss where worldly concerns melted away. Think again, sucker! Our eternities will apparently be spent floating about pondering the intricacies of party politics and the efficiency (or otherwise) of semi-State bodies. The afterlife sucks.
More than anything else, though, Helen's Pearse seemed unexpectedly keen to play the part of an Oprah-esque self-help guru, dishing out crumbs of "inspirational" comfort. Moncrieff asked if he'd be willing to join them live on air, in case he had a message for those who were "anxious about the future of the country".
Nine agonising seconds of radio silence followed as Helen attempted to contact Pearse via the astral ethernet. "He's actually getting quite emotional," said Helen, finally. "He can't believe that this is happening."
He wasn't the only one.
We were told to "look to the past for the answers". We were reassured that "we have the ability". We were advised to "take the opportunities and go with them".
Fans of Pearse's sizzling graveside eulogy for O'Donovan Rossa probably felt their hero had gone downhill a bit, oratorically speaking. Platitudinous muck it may have been but, hey, give the old boy a break. He's 133. And, lest we forget, dead.
This was truly revolutionary radio. For far too long the voices of the privileged living have monopolised current affairs broadcasting. Nobody bothered asking the marginalised non-alive for their input. Surprising, really, because they don't require payment and you can force them to spout any old crazy crap. They're perfect guests... if you can tolerate ectoplasm stains on the microphones.
Speaking of unexpected voices popping up in unexpected places, the Pope is now on Twitter (@pontifex). Monsignor Paul Tighe (of the Pontifical Council for Social Communications) joined Mary Wilson on Monday's Drivetime to confirm that "the tweets will not be infallible", just in case you were wondering.
The inaugural papal tweets will, he explained, respond solely to "questions of faith". So quizzing him on the likely winner of X Factor will, alas, have to wait, for now.
Boo! You can always ask the ghost of Padraig Pearse, though. He's full of chat.