In “Conversations in Tusculum,” Richard Nelson’s deceptively well-mannered prequel to “Julius Caesar,” the sleepy rhythms of the titular hillside town lend themselves to hours of fishing and lounging and talking. Especially talking: Rarely has a play’s title been so accurate. Barely masked by the spirited debate, however, is the faint sound of the dogs of war straining at their chains. By the end of Mr. Nelson’s mildly pedantic but nonetheless engrossing history play, havoc will be cried.

Until then, the discourse remains markedly sedate. The statesmen and orators wealthy and/or well-connected enough to own villas in Tusculum, located 15 miles outside of Rome, include Brutus (Aidan Quinn); his new wife, Portia (Gloria Reuben), and his brother-in-law, Cassius (David Strathairn). Once Cicero (Brian Dennehy) arrives, the stage is set for marathon rounds of lofty philosophical and political banter.

Context is everything, though, and one’s tolerance for this sort of talk shoots up in light of the cataclysmic events on the horizon. Mr. Nelson, who also directed, has set his play in the summer of 45 years before the common era, just a few months before the infamous Ides of March, with the intent of charting the moments at which despondency gives way to insurrection.

The juxtaposition of these churning passions with Tusculum’s pastoral charms (augmented by set designer Thomas Lynch’s rough-hewn scraps of furniture and Jennifer Tipton’s shimmering lighting) has an instructive allure; it’s as if Chekhov had filtered the revolutionary ferment of Shakespeare’s Rome through his own unforgiving prism of deep roots and even deeper torment. “Between the acting of a dreadful thing / And the first motion, all the interim is / Like a phantasma, or a hideous dream,” Brutus says in “Julius Caesar.” Even when the tangled lineages in “Tusculum” grow burdensome or the debates take on a scripted quality, Mr. Nelson’s gift is to turn this dream into a reality that is all the more dreadful for appearing so gradually.

In order to supply a satisfying level of conflict and resolution, he has modified Shakespeare’s conception of Brutus, backdating the young senator’s progress from dismay to paralysis to bloodlust. As the play begins, Brutus and Cassius are each licking their wounds after taking up arms against Julius Caesar during various ill-advised battles. This has presumably made for some strained family get-togethers: Brutus’s mother, Servilia (a suitably imperious Maria Tucci), is a former lover of Caesar, who is currently canoodling in Spain with Cassius’s wife. Portia, meanwhile, is the daughter of Caesar’s frequent enemy, the incorruptible Cato.

Romantic and Oedipal jealousy is hardly the only thing gnawing at these Romans’ consciences. There’s Caesar’s “Egyptian whore,” aka Cleopatra. There’s his duplicity when it comes to co-opting and manipulating potential threats, including Brutus and Cassius. (Seeing as how they might well have been executed for their past military transgressions, this quibble seems a bit churlish on their part.) And there’s his eagerness to have himself declared either Dictator for Life or “Divus Julius,” depending on whom you ask. Portia takes pointed note of this lack of humility: “So — it’s almost the end of ‘July.’ Isn’t that what we’re supposed to call it now?”

Cicero — who, on the strength of Mr. Dennehy’s vigorous and unexpectedly tender performance, rivals Brutus as the play’s moral and emotional center — finds his outlook affected by the recent death of his daughter and grandchild. “One could ignore things, accept them — because we were looking ahead for our children,” he explains. “But my child is dead. And what I am left with is what we have — not what we hope to become.”

The vitriol of the above speech may serve as a warning to those averse to modern-day parallels. For this Caesar is also a former drinker with imperial ambitions directed toward the Middle East and a decider’s disdain for intellectuals. (Shakespeare’s Caesar, you may recall, warned that Cassius “thinks too much: Such men are dangerous.”) Any similarities between him and current Western leaders are strictly un-coincidental; the back cover of the published “Tusculum” script touts the play’s “startling resonance with our age.”

Given the fate that befell Caesar less than a year after these events, following these parallels too far is both dramatically and morally dubious. But until a final speech throws Brutus’s shifting allegiances into sharp (and somewhat abrupt) relief, Mr. Nelson has succeeded in taking the pulse of a benumbed population on the brink of shaking itself into action.

He is less successful at breathing comparable life into all of the performances. Mr. Quinn’s fidgety, Method-heavy approach, with its anguished pauses, clashes with Mr. Strathairn’s affecting naturalism and Mr. Dennehy’s ever-so-slight grandstanding. And Ms. Reuben’s and Ms. Tucci’s roles are too sketchily written to allow them more than the occasional glancing blow. (The sixth and final character, an itinerant actor named Syrus, was crisply played by the understudy, Jeremy Strong, at a recent press performance.)

Cicero, still grieving over his terrible loss, spends much of his time in Tusculum brooding in the darkened rooms of his villa. When Brutus offers to shut the blinds after their latest talk, the philosopher declines. “Once the world’s in,” Cicero says, “you can’t get it out.” With characteristic civility and precision, the prolific Mr. Nelson has charted how the chaos of the world enters in the first place: slight by slight, betrayal by betrayal, injustice by injustice, conversation by conversation.

Until March 30 (425 Lafayette St. at Astor Place, 212-967-7555).