If the main players in “Churchill” took the stage in London or New York or Chicago and executed their roles exactly as written and performed in this film, I’d want to see that.

On the stage, it could be a powerful and moving work. As a movie, it’s a sometimes effective but more often tedious history lesson in which the legendary Winston Churchill at times comes across as a petty, self-consumed contrarian who always regards everyone in the room as his strategic, moral and spiritual inferior, regardless of the circumstances and the nation of the debate.

And when that room includes the likes of Gen. Dwight D. Eisenhower and the debate centers on Operation Neptune, aka the Normandy Invasion, such assumptions on Churchill’s part are not always correct.

The commanding Brian Cox is a natural choice to play Churchill. Cox slips comfortably into the Churchill wardrobe and props, as familiar as any superhero costume: the signature Homburg hat, the three-piece suit, the bow tie, the ever-present cigar, the walking cane, the booming voice, the fondness for scotch, etc., etc.

Unfortunately, the director Jonathan Teplitzky and the screenwriter Alex von Tunzelmann focus on just the four days leading up to D-Day and in particular Churchill’s obstinate resistance to the Allied invasion of the French coast, even though plans for the invasion have been in motion for more than a month, and even though Churchill’s own military command is on board with Ike’s strategy.

John Slattery, best known as the the silver-haired sly fox Roger Sterling on “Mad Men,” might seem a curious choice to play Gen. Eisenhower, but it’s a fine and steady performance in a somewhat underwritten role.

Time and again, we get scenes of Churchill exploding in a rage-filled diatribe about how thousands of lives will be lost, and how he witnessed the horrors of combat carnage in the first World War, and how this plan is madness. He lectures but doesn’t listen, shouts but does not contemplate.

And time and again, Churchill is told this is a different war, and the generals and field marshals planning this invasion are well aware of the risks, and it’s really not the prime minister’s duty to stand in the way of the mission. You’re still fighting the last war while we’re trying to win the current one, says Eisenhower.

At which point Winston stalks off and takes out his frustrations on his long-suffering wife (Miranda Richardson) or his worshipful young secretary (Ella Purnell).

The filmmakers reference Churchill’s battles with depression and his increasing reliance on drinking, but in overwrought tones. He screams bloody murder at his secretary because she didn’t double-space a memo; he violently swipes the breakfast dishes to the floor when his shaking hands betray him; he prays with feverish madness for a torrential storm to come raining down to postpone the operation; and he’s so self-pitying and combative with his wife that when she finally slaps him across the face, we wonder what took her so long.

History tells us Winston Churchill was one of the most influential and admired figures in modern British history. In “Churchill,” he seems consumed with establishing just such a legacy.

The problem is, the man we see onscreen only rarely comports himself like one of the greats. Far too often, he actually seems like the smallest man in the room.

★★

Cohen Media Group presents a film directed by Jonathan Teplitzky and written by Alex von Tunzelmann. Rated PG (for thematic elements, brief war images, historical smoking throughout, and some language). Running time: 98 minutes. Opens Friday at local theaters.