And they’re singing their lines because...?

Forget Betty Buckley’s purring rendition of “Memory” in “Cats” or Patti LuPone’s royal treatment of “Don’t Cry for Me Argentina” in “Evita.” Great as they were, the showstopper to end all showstoppers -- at least for those of us too young to have seen Ethel Merman belt out “Rose’s Turn” in “Gypsy” -- was Jennifer Holliday’s cyclonic “And I Am Telling You I’m Not Going,” the bruising R&B; aria that brought the curtain down on the first act of the 1981 Broadway musical “Dreamgirls.”

If ever there were a reason that the show was unimaginable as a film, it was this number. How could anything compete with the radiant memory of Holliday’s staggering cri de coeur?

Naturally, the question on everyone’s mind is how Jennifer Hudson, who plays Effie White in the new “Dreamgirls” movie, comes through in her big moment.

The answer, surprisingly enough, is remarkably well. But more important, the film, directed and adapted by Bill Condon, works because it embraces its own medium and doesn’t try to pass itself off as a theatrical production for the multiplexes.

Alan Bennett’s “The History Boys,” another high-profile stage-to-celluloid transfer this year, has a harder time making the transition. Interestingly, the histrionic classroom scenes in which the boys act old Hollywood melodramas, improvise brothel bargaining in French and sing cabaret torch songs are the most memorable. It’s the more straightforward dramatic bits that seem embarrassingly stagy.

Realism in film isn’t easy to get around, and movie musicals have the toughest challenge. No matter how you spin it, breaking out into song and dance isn’t normal behavior outside of a club, a disco or the privacy of your curtained home. For most of us, launching into a power ballad on the job would probably result in not having one to go back to in the morning.

The first thing the director of a movie musical has to figure out is how to construct a cinematic world in which communicating in music doesn’t seem, well, totally nuts. Last year’s “Rent” pretended it didn’t matter -- that we would accept a documentary-real East Village with denizens erupting, en masse, into rock anthems.

Not quite. Strangely, the stage version didn’t even try to get away with that. The Broadway set screamed concert musical, not Alphabet City. The mood was urban, but the address was purely -- and contemporarily -- theatrical.

Adaptation, to paraphrase Sondheim, isn’t easy. Just because the camera can take us anywhere doesn’t mean it should. On the other hand, one needn’t go to such surreal extremes as Baz Luhrmann in “Moulin Rouge!” to figure out how to switch between story and song. “Chicago,” the most successful movie musical since “Cabaret,” found a solution by cleverly accentuating the characters’ showbiz connections and then using dreams to help them break out of their prison cells, which can certainly put a cramp in an inmate’s 11 o’clock number.

Generally speaking, backstage musicals -- harking all the way back to “42nd Street” -- have the best luck as films. And Condon, who wrote the screenplay for “Chicago,” wrings as much as he can out of the music industry settings in “Dreamgirls.” From the very beginning, at an Apollo-like amateur night in Detroit (kinetically enhanced by the crack theatrical lighting team of Jules Fisher and Peggy Eisenhauer), the film pulses with the vitality of live performance. We feel the nervous energy of the Dreamettes backstage and can see the sweat on the other entertainers, who are working the crowd into a roaring frenzy.

Henry Krieger and Tom Eyen’s score structures the story as much as it flows through it. The action moves from theaters to rehearsal rooms to recording studios to piano bars -- environments in which all the singing-icon wannabes, including Eddie Murphy’s James “Thunder” Early, a hyperactive hybrid of James Brown and Marvin Gaye, are perfectly at home.

To open up the dramatic world, Condon’s direction frequently takes flight into the fantasy realm of music video, with the usual visual cliches of cityscapes at night, rambling highways and romantic ecstasy. “Steppin’ to the Bad Side” features Jamie Foxx and cohorts prowling through an alleyway that Michael Jackson could have moon-walked through in his “Thriller” days. When Foxx serenades Beyonce Knowles with “When I First Saw You,” the tenderness quickly gives way to a Vogue-style photo montage. Hackneyed choices all, but they keep the plot grooving along.

The good news is that the lyrical quality of the cinematography and editing allow one to enjoy the movie as pop entertainment. Not that there isn’t substance to the vision. Condon finds ample opportunity to introduce images of ghetto blight and riot turmoil, which provide counterpoint to the tale of a Supremes-like group’s rise from the projects into overly packaged stardom.

The bad news -- and there’s not too much of it -- is that the acting is rather flat. Hudson makes the deepest impression -- it’s always been Effie’s musical -- although her handling of dialogue isn’t half as impressive as her way around a song.

Yes, Holliday would no doubt blow Hudson off the stage in an “American Idol” runoff. Hudson has a phenomenal voice, don’t get me wrong, but she doesn’t have quite the same textured emotionalism or sustaining vocal beauty as Holliday. (But then, who does, other than perhaps Aretha Franklin in her prime?)

Still, I’ve seen the film twice, and both times I was moved to spontaneous applause at the end of Hudson’s rousing “And I Am Telling You,” which brought to mind Judy Garland’s heartbreaking directness. Clapping in a movie is silly if you’re doing it, annoying if other people are. But the ovation was loud and widespread. Hudson would make an ideal understudy for Holliday onstage, but she’s definitely -- listen up, academy voters -- the right actress for the film.

A great champion of “The History Boys” on Broadway, I find myself making apologies for the movie. It’s a respectable effort that showcases the rich talents of the original cast. But it lacks the intoxicating charm that allowed Bennett’s dramatic curio to vacuum up all the most prestigious theater awards in London and New York.

Too often, one has the impression of an awkwardly filmed play. Nicholas Hytner’s direction has a claustrophobic quality that wasn’t in evidence in his less physically expansive staging. Tight closeups are unable to absorb some of the farcical shenanigans, and the prosaic interactions of the boys at school appear unnecessarily stilted.

But the crux of the matter is that Bennett should have reworked the script so that more of the theatrical delight would have been preserved along with the impressive sweep of ideas and the fine performance of Richard Griffiths as the gifted, wayward teacher trying to instill in his pupils some sense of the authentic value of art and literature.

One memorably droll moment from the Broadway staging, impossible to straightforwardly reproduce in the movie, suggests the inevitable loss that happens in translation from one medium to another. Frances de la Tour, who plays the sober-minded veteran on the faculty, stepped before the audience and announced, in a voice as dry as English tea biscuits: “I have not hitherto been allotted an inner voice, my role a patient and not unamused sufferance of the predilections and preoccupations of men.”

The line brought the house down. Surely, Bennett could have found some equally daring cinematic analogy. If backstage musicals can manage so well, why not an unorthodox comedy of ideas?

charles.mcnulty@latimes.com