In the 1980s, when the BBC arts department spent several years putting together a history of the theatre (presented by Ronald Harwood), it was worth phoning the production office just to hear the unenthusiastic tone of the person answering. He had evidently not left his desk in three years. “All the World’s a Stage,” he sighed. At the time, it seemed hilarious. But, then again, perhaps we bought our kicks too cheaply in those innocent, far-off days.

I mention it because I’ve recently realised how little enthusiasm I’ve been putting, over the past few months, into the response, “Oh, just dramatic monologues”, when people asked me what I’d been writing. I’ve written six half-hour monologues for Radio 4, you see. And the problem with working in one form for such a long period is that you forget how special it is. You also forget that other people may not know immediately what you mean. “Will you be reading them yourself?” people ask. And I have to say no, these are dramatic monologues, for actors. “Like Alan Bennett’s Talking Heads” is the only explanation that always works – although obviously it then makes people wonder why you would bother writing dramatic monologues like Alan Bennett’s Talking Heads, when Alan Bennett has, clearly, written them already. And I don’t mind admitting, that’s not a bad point.

It turns out, however, that category confusion is quite reasonable around the words “dramatic monologue”. To student actors, any speech longer than five lines can be found anthologised as a “monologue” for audition purposes. Many people with only a vague interest in the theatre assume that the monologue has something intrinsically to do with the vagina. Meanwhile to critics of English literature, the dramatic monologue is specifically a kind of poem popular in the mid-Victorian period. Robert Browning’s “My Last Duchess” (1842) is the most often cited example. “That’s my last Duchess painted on the wall, / Looking as if she were alive.” “My Last Duchess” impeccably follows the rules of its genre, having a narrator, an occasion, an implied auditor, an unfolding of narrative information (it emerges that the speaker has been responsible for the murder of his wife), and the cunning, gradual revelation of the speaker’s true nature.

Predictably, critics bicker over the definition of this genre — and there are some who even argue that the term “genre” itself has no substantive validity. What larks. However, the rules are helpful. And it transpires that the Victorians enjoyed this sort of poem because they loved the chance to climb inside someone else’s mind – especially the mind of a dangerous or insane person. Alfred Tennyson’s bestselling Maud (1855) was an extremely long “mono-drama” on the theme of madness, and he was famously addicted to it. He read it aloud whenever he spotted the opportunity. “I hate the dreadful hollow behind the little wood,” he would begin, and people who had timed their exit badly just had to take their coats off and settle back down for the duration.

Where the literary critics go a bit quiet, it seems to me, is in relating this type of monologue to the sort that actually happens on a stage – either in serious drama or in comedy. There are whole (short) plays by great playwrights, after all, written as monologues. And when you look at them, they are likewise often concerned with deviance: for example, Samuel Beckett’s 1972 play Not I, with its darkened stage and blacked-out auditorium, and just Billie Whitelaw’s madly muttering mouth picked out by a spotlight. August Strindberg’s The Stronger (1889) is a characteristically nasty piece of work involving two on-stage female characters, one of whom never speaks, battling for supremacy. Harold Pinter’s television play Monologue (1973) wastes no time getting to the worrying stuff: “I think I’ll nip down to the games room. Stretch my legs. Have a game of ping-pong. What about you? Fancy a game? How would you like a categorical thrashing?”

The thing about monologue is that it’s immediate. It happens now. It happens here. And it is literally “im-mediate”, in that there is ostensibly no mediation: nothing intervening between the character and the audience. That’s why, in certain magical theatrical circumstances, it can seem to fill the world. Everyone can think of a moment in a great play when attention zooms in on a single character telling a story – and the audience simply stops breathing. It happened memorably in Conor McPherson’s The Weir (1998), for example, as each of the characters told their tale; and in Bryony Lavery’s Frozen (1998, 2002). It’s a famous reward at the end of Eugene O’Neill’s The Iceman Cometh (1940); and in Pinter’s The Caretaker (1960), with its pivotal speech from Aston about the electro-convulsive therapy, it provides one of the best moments of western theatre.

“He showed me a pile of papers, and he said that I’d got something, some complaint. He said … he just said that, you see. You’ve got … this thing. That’s your complaint. And we’ve decided, he said, that in your interests there’s only one course we can take […] he said, we’re going to do something to your brain.”

Elsewhere on the non-legitimate stage there have always been monologues, too. Variety has thrown up many brilliant monologists, from Dan Leno to Joyce Grenfell, from Robert Benchley to Bob Newhart, to Whoopi Goldberg. I am a great fan of Michael Frayn’s brilliant dinner-party spitting monologue (“Do you spit? No? You don’t mind if I do?”) But two things separate all these earlier types of monologue from the Alan Bennett talking head: they are generally addressed to a particular person (“Is dis not a smeshing cocktail party? I am so fond for a cocktail party,” begins one of Joyce Grenfell’s); and, more important, they do not move on in time. The critic Dorothy Mermin wrote in 1983, “Monologue lacks the resources to develop the temporal dimension, the notion of life as a continuing process of growth and change.” But within five years of this party-pooping pronouncement, Alan Bennett had disproved it. It was a simple solution. He wrote his monologues in scenes.

When I decided to try my own hand at monologues for the radio (for the first series of A Certain Age, 2002), I adopted the Talking Heads model quite unthinkingly, assuming it was of classical origins. And surely the multiple-scene, talking-to-nobody-in-particular structure was a natural way to tell a story, being the dramatic equivalent of the video diary? However, I now realise that when the first series of Talking Heads was written in 1988, it was probably revolutionary. It added a crucial fourth dimension to a well-established form. Alan Bennett is, arguably, the Doctor Who of the monologue.

They are bloody hard to do, by the way. They wear you out; wring you dry. Getting inside your characters’ heads and manipulating their lives entirely from within requires a muscular imagination, to say the least. It’s like Being John Malkovich, but thankfully without all that exaggerated stooping. I’ve now tried monologues with twists as well as ones that are more concerned with exposition. I’ve tried light ones and deeper ones; redemptive and non-redemptive. It’s astonishing how many things you have to think about. Supremely, you must continually ask yourself, “How far does this person know what’s really going on? Do I know, either?” Bennett said his talking-head people “don’t quite know what they are saying, and are telling a story to the meaning of which they are not entirely privy.” Well, typical of him to make that sound easy, too.