IT WOULDN’T be hard to parody Jonson’s Sejanus, whose cast includes not only many anonymous lictors, tribunes and so on but 28 named Romans ranging from Macro, who is very formidable, to Minutius, who is so insignificant that he passes unnoticed. “Is’t Caligula I see skulking behind yon column?” “Nay, ’tis Satrius Secundus with Pinnarius Natta in tow.”

And when aeons ago I read the play, blinking at its plunderings of Tacitus and Juvenal, I recall having to fight another ancient personage, Morpheus, the god of slumber. But, astonishingly, Gregory Doran and his RSC sub-company combine dramatic momentum with clarity, ensuring that we soon distinguish Nattas from natterers. Before long we see why this forgotten tragedy got the Catholic Jonson into trouble with the Privy Council when he published it in the edgy London of 1605. The most noble characters aren’t necessarily open anti-monarchists, but some hanker for Brutus and Cassius, and all despise an emperor who, like James I, had his flatterers and hangers-on, spies and informers.

The principal of these is Sejanus, the low-born soldier whom Tiberius came close to making his heir. At Stratford, he is the cock of the walk in his Praetorian Guard finery and blond ponytail, standing out from the senators in their dull oatmeal togas and, as played by William Houston, radiating a smiling confidence that scares others. Just before the “fall” of the title, he is on a throne above a seething, maggoty mass of obsequious dotards — and, eyes fixed serenely upwards, clearly expects to be carried by swanmobile to Olympus.

The folly of hubris is the play’s subject, but so is the fear, self-interest and corruption of politicians. Only a few — primarily Geoffrey Freshwater’s passionately upright Silius and James Hayes’s doggedly decent Sabinus — resist the murderous Sejanus and his lackeys.

But those of them who survive are either ineffective or (in Macro’s case) ruthless, for this is a play brimming with Jacobean pessimism, whether it treats the aristocrats or the pleb who worships Sejanus’s statues, then tears him to bits, then regrets its own violence.

From time to time the verse gets richer, more intense, reminding you that Jonson was about to write Volpone. You also feel that the dramatist takes a certain relish in Sejanus’s own jubilant excesses, which here include surreptitious buggery that hastily ends when the emperor enters saying: “Is yet Sejanus come?” But there is no denying that, apart from Barry Stanton’s sly, devious Tiberius, the role Shakespeare allegedly played, the rest of the characterisation is thin, obvious. Jonson was a wonderfully robust comic writer but, as Sejanus inadvertently suggests, not a great tragedian.

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