The joke is not so much that a man gets pregnant, but that the man is Schwarzenegger Thompson has spoken of a heavy schedule, but there's not much evidence on scree n
WHAT ARE the British Board of Film Censors doing keeping Natural Born Killers under armed guard, when they allow Junior (PG) to stalk the land? There is a scene towards the end of Junior (regard this as a warning rather than a giveaway), in which Emma Thompson has sex with a seven-months-pregnant Arnold Schwarzenegger. The pictures are anatomically graphic: the image of Arnie's bulging belly may forever haunt your dreams, floating around them like the deserted spaceship in Alien. It is all done i n the best possible taste - an affectionate, lumbering pas de deux - except that by now taste has fled, hands held up in horror, vowing never to work in Hollywood again and to sack its agent. Maybe these ante-natal manoeuvres are what the BBFC means by P arental Guidance.

Junior is what is known in the business as a high-concept comedy, which usually means low-grade gags. Schwarzenegger and his director, Ivan Reitman, worked on another physiological hypothesis in Twins, where Arnie and Danny DeVito played totally unidentical twins, results of a scientific experiment. The cinema is fascinated with genetic engineering, perhaps finding in it a reflection of its own flaunting of the laws of nature - the notion that anything can be fixed in the lab. The gag here is that scientist Dr Alexander Hesse (Schwarzenegger) impregnates himself with a fertilised egg - don't ask how: the idea is clearly ova-the-top - and goes all the way with it.

The joke is not so much that a man gets pregnant, but that the man is Schwarzenegger. An icon of masculinity is melted down into a mush of emotion. (Such are the polarised gender stereotypes the film deals in.) The Arnie of the early scenes - erect, narcissistically tanned, severe - is the model of a high-flying medical researcher. His technocratic language (he talks of "invasive human procedures") contrasts with that of his friendlier, furrier colleague, Danny DeVito ("You gotta let us try this on realwomen"). An early dream sequence almost reprises the scene in Kindergarten Cop where the horrified he-man is overrun by mewling infants. Kids are his worst nightmare. Reitman even has one of them wet himself on Arnie, confirming h is own reputation as Hollywood's limbo dancer - he'll stoop to anything in the hope of a laugh.

The movie gets a lot of crass comic mileage out of Arnie's gradual transformation: the throwing-up fits, the puzzlement of colleagues at the glow in his cheeks, his burgeoning waistline and diminishing mobility, his complaint that his "nipples are very sensitive". Eventually Arnie embraces femininity, though not without an almighty, if predictable, hormonal struggle: "I feel so humiliated. I feel like I've lost control over my body."

You can see what attracted Schwarzenegger and Reitman to this idea - and why they are so ill-suited to carry it out. They are both control freaks. Reitman is above all an orderly director - you probably have to be to have made movies that have grossed $1billion between them. There is no mystery to his success. His films are extremely carefully crafted. There is a shot early on in the laboratory, as Arnie crouches on the floor, that frames the three leading characters, with their heads in an ascending line, Schwarzenegger at the bottom and Thompson at the top. It is the sort of comic visual emblem for the movie's theme - role-reversal - that Reitman specialises in. When Arnie takes the experimental drug, "Expectane", which will e nsure his pregnancy, astorm rages outside, like a Shakespearian symbol of an upheaval in nature.

But Reitman's rigour is joyless. He never seems to let himself go, as the great comic directors do (or seem to). It is a particular problem here, with a story which ought to flirt outrageously with the surreal. When we see Schwarzenegger in a pink blouse, bobby socks, a pearl necklace and a wig that summons up the ghost of Norman Bates's mother, we know we are in very strange territory indeed. It needs a Bunuel or an Almodovar to steer us gaily through it - anybody but Reitman and his billion-dollar pragmatism. Emma Thompson provides some ditsy English charm, as the dowdy, absent-minded, expatriate researcher. She's the sort of girl who can hold a conversation with you without realising she has a piece of cheese stuck to her cheek. In interviews, Thompson has spoken of the heavy shooting schedule, but there's not much evidence of it on the screen: the part is a throwaway, extending her range without ever stretching her talents. Not even she can escape the strait-jacket of Reitman's comic design.

Junior has disappointed at the American box office, and you can see why. Reitman has messed up the formula of his Schwarzenegger hits. The joke then was that a softer Arnie (a gentle scholar in Twins, a primary-school teacher in Kindergarten Cop) eventually revealed his ferocity. It rang satisfyingly true to see the wolf throw off his sheep's clothing. Here the wolf turns into a sheep, which is neither believable nor amusing. And by not acknowledging how odd it is, Reitman has given us a film that is grotesque as well as stupid.

You are much better off going to see That's Entertainment III (no certificate), which has a limited run at the National Film Theatre. If the standard of the extracts in this MGM dance compilation is not up to that of the first two films, it is merely a slip from celestial to sky-high. This is an astonishing hoofing extravaganza, with Judy Garland, Gene Kelly, Lena Horne and Fred Astaire among, as MGM publicity used to say, a galaxy of stars. You see not only that they don't make them like that any more,but how they made them like that in the first place. Split screens are resourcefully used: to show Eleanor Powell tap-dancing on one half, and the camera frantically tracking her on the other; to compare Joan Crawford and Cyd Charisse dancing to the same production number ("Two-Faced Woman") in different films - and to make it clear that the wrong one (Charisse in 1953's Band Wagon) landed on the cutting-room floor.

These musicals were mainly about sex, reckons Gene Kelly, one of a number of star narrators. And seeing Charisse leggily stretching the confines of a grey dirndl skirt or Lucille Ball taming a pack of exotic "cat women" in spangly open-backed dresses, you have to agree. Esther Williams, "Hollywood's Mermaid", reveals that "MGM were committed to finding new ways of getting me out out of my clothes and into the pool". Best of all is a clip of Gene Kelly's soft-shoe shuffle duet with a sheet of newspaper, from Singin' in the Rain. Genius like that still deserves to make headlines.

In Above the Rim (15) basketball comes close to dance. The sequences of the players warming up - jumping, one after the other, to keep the ball bouncing against the back-board; rehearsing lightning passing movements - have a fluid choreographic beauty. The script is conventional sporting melodrama - a promising athlete caught between good and bad influences. But, like Hoop Dreams, which I wrote about when it showed at last month's London Film Festival, and which is now being talked about as a possible first documentary to gain an Oscar nomination for Best Film, it offers an insight into the heavy pressures and slim opportunities of young black American lives.

The Punk and the Princess (18) is a likeable, sometimes laughable comeback by Mike Sarne ("The best director I ever had" - Raquel Welch). A punk Romeo and Juliet, it contrasts the velvelty images of a stage version of the play with the squalor of the real world, in which a young run-away falls for a wealthy American girl. The links to Shakespeare are tenuous. But just when the whole ramshackle structure looks like collapsing, Sarne pulls it together - especially in a smart parallel to the Tybalt fight, in which the hero and a sharp-suited yuppie fence with pool cues, and in an arch ending.

Cinema details: Review, page 98