Can an agnostic liberal Democrat find lifelong friendship with a churchgoing conservative Republican? Suspend for now the cynicism of your Facebook feed and consider the example of Henry Fonda (Democrat) and James Stewart (Republican), who held it together for a good half-century without unfriending each other or wondering out loud how they would get through the holidays or coming to blows over immigration policy.

(Simon & Schuster)

What was their secret?

Start with this now-inconceivable fact: They never spoke of politics. Or, for that matter, work. Or war or women. They built kites and model airplanes and, when the mood struck, conceived elaborate practical jokes and, together, found the spirit of relaxation and fun that often eluded them as individuals. To borrow the title of Scott Eyman’s smart, generous chronicle, they became “Hank & Jim,” a pair of guys who asked nothing of each other but propinquity.

And, of course, if they had been simply Hank and Jim, we wouldn’t plunk down a nickel for their story, however much we applauded their political high-wire act. But they did something even more unaccountable, which was to stumble into the same revelatory method of movie acting — a carefully calibrated, jealously guarded “non-style” that favored withholding over display and had the surprising effect of making their work more durable and timeless than their heavy-lifting peers. Which is to say that Mr. Smith and Tom Joad will live longer than anything created by Paul Muni.

Did the two men share theory and practice along the way, or were they each other’s accidental bystanders? That Eyman fails to answer the question may only mean that it’s unanswerable. We know that Fonda was far and away the pricklier of the two: a cold, coiled and reticent man given to deep silences and, in the words of daughter Jane, “Protestant rages.” “I cling to my angers,” he once admitted, but he clung rather less to his first four wives — the fifth was the love match — and drank rather too much and never overcame the essential shyness that drives so many actors into the arms of their own characters.

Stewart, by contrast, grew up in the cunningly concatenated small town of Indiana, Pa., where his father owned the hardware store. Without a great deal of angst, he switched his career path from architecture to theater and became, according to one early admirer, “the least neurotic actor I’ve ever been around.” He married late, but it was for keeps, and his latter-life credo, as boiled down by Eyman, was “one wife . . . one house . . . two dogs . . . one car . . . one diet.”

Both men were loners, though, and both embodied “integrity mixed with a bloody-minded obstinance that wasn’t acting.” Crucially, too, both men had their hearts stomped on by the same woman: the willowy and incandescent Margaret Sullavan, who co-starred with Stewart in four movies and remained married to Fonda for something like 20 minutes. (Neither man seems to have gotten over her, and neither will you if you watch “The Shop Around the Corner.”)

It was World War II duty, perhaps, that accentuated the two friends’ fundamental differences. Fonda opted for naval intelligence work in the South Pacific, where “the strange disassociation of fighting a bureaucratic war without ever firing a gun” would shape his portrayal of Mister Roberts, a man of action in an idled world. Stewart, in his usual laconic fashion, went where the danger was, serving as lead pilot on 19 bombing missions and “learning to live with the fact that you and your crewmates could be highly skilled, do everything right, and still die.”

Author Scott Eyman (Greg Lovett)

Far from stopping him up, the experience of battle enlarged his acting in unsuspected ways. Despite what you’ve been told, his first post-war movie, “It’s a Wonderful Life,” is not a feel-good holiday classic but the unsparing portrait of a man coming apart at the seams. (Eyman argues that “the gradual entrapment of George Bailey” suggests what Stewart’s life would have been had he never left Indiana, Pa.) And over the ensuing 15 years — in genres ranging from Western to thriller to comedy — Stewart found a way to show the darkness lapping just beneath his nice-guy mannerisms.

Did he leave his old friend behind in the process? Eyman makes an equally eloquent case for Fonda’s art: the “instinctive austerity,” the “pointillist technique” that weds “inner stillness” and “vocal urgency,” the way in which the actor’s own walk “works against the flow of life around him.”

That stiff outlier gait may have expressed more than he knew. A black friend of James Baldwin’s, after seeing “The Grapes of Wrath,” insisted that its star had “colored blood.” “White men don’t walk like that!” declared Baldwin’s friend as he imitated Fonda’s “stubborn, patient, wide-legged hike away from the camera.”

Louis Bayard is a novelist and reviewer. His most recent book is “Lucky Strikes.”

Hank and Jim
The Fifty-Year Friendship of Henry Fonda and James Stewart

By Scott Eyman

Simon & Schuster. 367 pp. $29