LONDON – You needn”t have seen the 1964 Disney family staple “Mary Poppins” — though I shudder to think, almost 50 years after its release, of a childhood completed without it — to be familiar with the practically perfect English nanny”s all-purpose maxim that “a spoonful of sugar makes the medicine go down.” It was a line conceived not by P.L. Travers, the famously prickly Australian author of the children”s books behind the film, but by Richard and Robert Sherman, the Disney studio”s in-house songwriters.

Intentionally or otherwise, it was a cannily appropriate bit of invention: in a sense, it neatly sums up the Disney ethos of using whimsy and cheer to make life lessons more palatable to young viewers. (Or older ones, for that matter.) Disney, after all, was the man who changed the definition of “fairytale” in the public imagination from Grimm-dark allegory to one of mandatory happy endings. Travers, for her part, liked the medicine.

“Saving Mr. Banks,” John Lee Hancock”s bright, entertaining and — inevitably — somewhat selective overview of “Poppins'” conflict-laden journey to the screen, is a film that aims for the inverse of that formula: a small dose of acrid personal history is applied to make its sentimental study of creative collaboration yielding personal catharsis that much easier to swallow. That’s not necessarily a knock against it. If the tidy emotional geometry of Kelly Marcel’s script occasionally feels Disney-esque, that seems only right for a film explicitly about the pervasiveness of Disney”s optimistic storytelling principles in popular culture — and more implicitly about the way even those heightened principles can mirror the odd human truth. Sometimes life is sentimental, and some will fight it more than others.

As played by a hilariously clipped, unaccommodating Emma Thompson — cutting a rigid figure with her poker-like posture and steel-wool hairdo, seemingly sewn from birth into a tweed skirt suit — Travers fights that fight with unflagging conviction, protecting her creation in order, as the film”s honey-lit flashbacks make increasingly clear, to protect her own memories. Art adapts life, and life adapts art, several times over in “Saving Mr. Banks,” and not just for Travers.

The uptight Australian and the ruthlessly twinkly Walt Disney (Tom Hanks, striking the necessary balance of magnetism and Teflon lack of affect) engage in a civil war for artistic custody of “Poppins”; it”s so hard-fought, we learn, because they’ve both built their own fantasy empires upon a foundation of personal hardship and a whole heap of daddy issues. Naturally, they can’t get along because they have so much in common: it’s practically a romantic comedy without a hint of sexual want. (Well, it is a Disney movie, after all.)

“I won’t have her careening toward her happy ending like a kamikaze,” barks Travers to her put-upon agent, as he approaches her for the umpteenth time in 20 years with Disney’s request for the screen rights to her international publishing phenomenon. In 1961, she finally acquiesces to meet the mogul — she’s 61, short on both inspiration and finances — on the further condition that no part of the film be animated. We all know that Disney would break at least one of those commitments: toward the end of the film, Thompson watches the completed “Mary Poppins” for the first time, and her woebegone expression as dancing cartoon penguins fill the frame is a treat.

But did Disney give Poppins herself a happy ending? Arguably not: played to Oscar-winning effect by Julie Andrews, she remains one of Disney’s most sinuous, even sinister, heroines, floating unceremoniously off the screen and leaving the Banks family to their healed devices. It’s a bittersweet conclusion that came about as a result of Travers’ stubborn script demands at the pre-production stage, made while she held her unsigned contract as collateral. The most enjoyable stretches of “Saving Mr. Banks” aren’t, in fact, her terse tête-a-têtes with Disney, but the tortured workshop sessions between the author, screenwriter Don DaGradi (Bradley Whitford) and the Shermans — delightfully played by Jason Schwartzman and B.J. Novak as stooges with a wily streak, as clean-scrubbed and sharply side-parted as a pair of Disney princes.

Travers’ passive-aggressive manipulation of the script to reflect her worldview is not entirely in the interests of cynicism, as she insists upon the redemption (yes, the “saving”) of the film’s unadvertised protagonist, proud English patriarch Mr. Banks — written by DaGradi as a cold prig, and more forgivingly by Travers as a version of her own troubled but devoted father, Travers Goff. Travers may be a maddeningly unreasonable presence in these script sessions, but she’s not a misguided one, as she steadily makes herself heard, enriching a film she still loathes in principle. (She makes some allowances, too, as her resistance to the film’s musical aspects is thawed by Shermans’ infectiously rousing closing number “Let”s Go Fly a Kite”: she”s not a zombie, dammit.)

Thompson’s cunning performance revels in Travers’ withering contempt for all enablers and opponents alike, like a fussier Transatlantic take on Miranda Priestley. However enjoyable, this haughty damedom runs the risk of wearing thin, but the actress suggests enough of the woman’s tightly leashed inner demons to render the film’s frequent flashbacks to Travers’ Outback childhood turgidly on-the-nose by comparison.

That may partly be the fault of a sorely miscast Colin Farrell as the devil-may-care alcoholic Papa Goff, whom the actor elects to play as Johnny Depp’s J.M. Barrie with added man-sprite creepiness. But it’s Marcel”s script that loses its zip and focus in these scenes, resorting to blunt symbolic cues and contrived doubling to convey in shorthand an idyllic childhood gone awry. A climactic scene intercutting the composition of Banks’ number “Fidelity Fiduciary Bank” and Goff’s public humiliation at a country fair is one of the few formal reaches in Hancock’s otherwise workmanlike direction, and falls badly flat.

This rather literal extraction of pathos from the material eventually bleeds into the film’s present, crucially in a lengthy monologue for Disney wherein he makes a last-ditch attempt to woo Travers by laying out the parallels between their respective backstories in exhaustive detail. Hanks delivers it well, maintaining a venal, self-serving edge to a pitch Don Draper would be proud of, but it’s hard to tell just how emotionally resonant Marcel intends it to be, not least because the scene professes to lay bare the defenses of two characters the film keeps largely hidden to the very end. Aside from one veiled allusion to estranged family, “Saving Mr. Banks” makes no mention of Travers’ troubled relationship with her adopted son, nor her reputedly fluid bisexuality. Given the Disney remit, it’s understandably more convenient to present her as a kind of spinster-aunt figure, though considering how much the film is predicated on the notion that Travers wrote — and Team Disney rewrote — her life through her art, these seem significant omissions.

Yet “Saving Mr. Banks” charms in spite of its movie-world airbrushing, a process that extends all the way from character arcs to costume. (This is the kind of painstakingly finished, beautifully dressed period piece where no one’s clothes look worn for longer than the length of the shot: that Farrell’s white banker’s collars remain so crisply laundered through the sweat and dust of rural Down Under is a trick worthy of Poppins herself.) Perhaps it charms because of that very artifice. If any film has a free pass to Disney-style distortion of reality, it’s surely a Disney movie about the creation of a Disney movie. “Mary Poppins,” after all, is a great film that resulted from considerable infidelity to its source: P.L. Travers was right about some things, and Walt Disney about many others.

It’s tempting to wonder what both fiercely protective artists would have made of “Saving Mr. Banks,” a respectably lesser film that takes their hostile but ultimately magical collaboration as its own happy ending.