'Nasty piece of work' or a journalist just doing his job?

Once upon a time, and in black and white, TV journalists 'invited' ministers and other leading politicians to 'comment' upon this or that event.

Then, in 1959, and on ITN, Robin Day 'confronted' the Japanese Foreign Minister about copycat ball-bearings. Japanese industrialists were building up an unenviable reputation for manufacturing espionage and Robin bit the bullet (or ball-bearing). They had copied a UK product and produced it more cheaply.

Robin's interrogation was robust and persistent, though not rude. Life was never the same again.

The soft-shoe shuffle was gone and the blunt, yet courteous, engagement was born.

Watch: Boris squirms on PM talk: 'It's not going to happen'

Today on the BBC's Andrew Marr show, Eddie Mair "put it to" Mayor Boris Johnson that the former MP for Henley, and presumed would-be Tory leader, was "a nasty piece of work".

Mr Mair supplied chapter and verse of alleged falsehoods. He brought up the infamous telephone exchange with his friend Darius Guppy, in which Mr Johnson seemed to be willing to assist in what would have amounted to violence against someone Mr Guppy disliked, had it come to pass.

It was intriguing television, interesting both politically and personally. It both dominated and divided opinion on social media for quite some time. It will feature in tomorrow's papers, spun according to the political interests of the publisher and writer.

It brings to mind the edict of H.L. Mencken that the relationship between a journalist and a politician should be that of a dog and a lamp post. It also reminded me of the much-quoted premise of many political interviewers when engaging with politicians: "Why is this lying b*d lying to me?"

The political interview is as much a forensic examination as it can be theatre.

Some, like Sir David Frost, appeared to take a 'softy-softly' approach, lulling the subject into a false and vulnerable sense of security. The 'quote' would drop, like manna from heaven, from the lips of the unsuspecting victim.

Others, like Jeremy Paxman and John Humphreys, are ferocious in their pursuit of truth - never letting up, frequently interrupting and, like fighting dogs, never loosening their jaw until the 'guest' lets slip what the interrogator felt the audience should hear.

It was the "nasty piece of work" phrase that stuck out for me.

Mr Mair is too experienced and professional a practitioner (he also presents BBC Radio 4's PM and occasionally BBC2's Newsnight) to have let such a phrase slip from his lips. Was it appropriate? Did it cross a line of firmness but courteousness?

Those of us who paddle in these treacherous waters would do well to ponder that question; as would those who get at least a part of their political intelligence from TV interviews, and who use them to distinguish their leaders from their would-be leaders.