In politics there is not a closed shop. The platform is open to anyone to have his say, to speak up, to participate. When the participant is a thespian force of nature such as Michael Sheen sparks are bound to fly. And so they have since the celebrated actor let rip in Tredegar, Wales, on Sunday, delivering an impassioned St David’s Day speech in defence of the NHS.

Sheen’s argument was a familiar one: namely that contemporary politics has been drained of honesty and authenticity, that “politicians are careful, tentative, scared of saying what they feel” and “all political parties drift into a morass of bland neutrality”. Quoting the founder of the health service, Nye Bevan, he declared: “We know what happens to people who stay in the middle of the road. They get run down.” All this has struck a chord, triggering a collective swoon on social media and elsewhere — especially his angry demand: “By God, believe in something.”

Let us allow that much of this is true, not to say commonplace. The detachment of the political class, the problem of popular disengagement, the bleaching of public discourse: all this is familiar territory, the stuff of Ukip leaflets, faux-furious blogs and hand-wringing documentaries.

What distinguished Sheen’s Bevanite outburst was, of course, the awesome power of the delivery. His Caligula (2003) and Hamlet (2011) are two of the more remarkable stage performances I have ever seen. On television and screen, he has made his name as an uncanny avatar for Tony Blair, Brian Clough and David Frost. He brings with him the dressing-up box of the giants he has portrayed.

Yet herein lies the hypocrisy: Sheen is the beneficiary of precisely the forces he deplores. There have indeed been moments when politics has risked becoming a branch of the entertainment industry — a phenomenon witnessed most vividly in the political careers of Ronald Reagan, Clint Eastwood and Arnold Schwarzenegger, but more subtly visible, too, in the rise of the politician as jester and game-show performer.

The border between the two worlds has blurred in TV series such as The West Wing, in which Martin Sheen (no relation) gave liberal America the Bartlet presidency as a consolation for the disappointments of the Clinton years. On Sunday, Michael Sheen was essentially playing Bevan, and with usual brilliance. But be in no doubt: this was showbusiness, not an ethical punctuation mark.

Consider, too, what he actually said. No mention, of course, of the ring-fenced NHS budget, and the recovery that has helped to make it possible. No recognition of the fact that the NHS has performed worse in Wales under Labour than in England under the Coalition.

Yet, to be fair, it was not that kind of speech. Sheen’s more culpable omission was this: the fact that the political culture of caution, focus groups and demographic calculation he so despises did not arise in a vacuum. It was the logical response to an electorate that demanded smooth, commanding leadership and broad appeal, rather than fiery passion and risk-takers. That other great Welsh orator, Neil Kinnock, was rejected twice at the ballot box. His rhetoric was often dazzling. But the nation stuck with the Tories in 1987 and 1992.

Then there’s Bevan’s famous line about the “middle of the road”. It’s still a zinger, and still completely remote from reality. The centre-ground remains the terrain of electoral victory, however much its compromises and grey zones infuriate ideologues. Does anyone truly believe that David Cameron did not win an outright majority in 2010 because he was insufficiently Right-wing, or that Ed Miliband increased his chances of becoming Prime Minister by promising to “bring back socialism”?

The worst line in Sheen’s speech was the one that went viral. “By God, believe in something” quickly acquired a Twitter hashtag and will surely displace “Je suis Charlie” as the T-shirt slogan du jour. Delivered by Sheen, it sounded thrilling, knitting neglected emotions back into the political fabric. But what does it actually mean? Is Sheen really saying that it is better to believe in something — anything — than to dither? Because, to put it mildly, what that other Celtic shaman, Yeats, called “passionate intensity” does not always have happy consequences.

The logic of Sheen’s position, if pursued, would be that “Jihadi John” — who definitely believes in something — is more honest than a party pollster. Let me emphasise: I am sure the actor didn’t mean this. When he referred to “belief”, he meant his beliefs, Bevan’s beliefs, the beliefs that underpinned the founding of the NHS. But conviction politics is not always cuddly and compassionate.

Nigel Farage “believes in something” — namely, that there are too many foreign speakers on his train and that mass migration makes him late for meetings. Enoch Powell “believed in something”, as did Sir Oswald Mosley. Are we to applaud such men simply because they hold, or held, strong convictions?

The timid agnosticism of many politicians today is indeed infuriating. But I’d rather have that than reckless populism and ugly atavisms awoken by self-proclaimed men of belief, not all of whom are decent, public-spirited thesps in donkey jackets.

I admire Sheen too much to tell him to stick to the day job. He has forced us all to think, which is no bad thing. And, as Allan Bloom wrote in Shakespeare’s Politics, the connection between public affairs and drama has always been rich, the political life “classically thought to be the stage on which the broadest, deepest, and noblest passions and virtues could be played, and the political man….the most interesting theme of poetry.”

So it can be, and an actor of great stature can illuminate this often-dreary stage. But he must expect no special privileges, or immunity from scrutiny. And the rest of us should not confuse a perfectly legitimate exercise in Welsh Tory-bashing (“Get out! Get out!”) for the emergence of a Cambrian Martin Luther King. In spite of the Tredegar drizzle, this was a flash of Shakespearean panache with a dash of Hollywood glitter. Do not be dazzled by the sheen.