One of the chief reasons Howard Hawks' Bringing Up Baby is the king of the screwball comedies because it never stops. It has a straight man in Cary Grant's hysterically put-upon paleontologist, but no one in the movie is at all normal. Hawks himself later criticized this after the film proved, amazingly, to be a complete flop, saying he should have had at least one person acknowledge the lunacy and provide some kind of sanity anchor. But that's what makes the movie so great: it never steps outside itself to note how ridiculous (and downright naughty) everything is.
Some of the first lines of the film are pure innuendo, showing Grant's David Huxley, framed in an unflattering, goofy Thinker pose trying to figure out where a brontosaurus bone goes and telling his fiancé "I think this one must belong in the tail." "Nonsense," she says, "you tried it in the tail yesterday." This sets the ball rolling on a flagrantly sexual movie that inverts gender roles, making Grant the creepily stalked object of affection of Katharine Hepburn, who flashes into the movie like a firecracker and only gets more spectacular from there. While David is out playing golf to woo a potential museum investor, Hepburn's Susan walks up and plays his ball. Then, she drives off in his car, dragging him along on the running boards. Take a deep breath, this is as calm as the film gets.
Grant had already established his comic persona that would serve him well for the remainder of his career, but he still plays entirely against type here. He's so good at being clumsy, using his acrobatic talents for the first time in film in service to magnificent pratfalls, that it's easy to forget that this is Cary Grant, sexiest man who ever lived and a powerhouse leading man. He submits entirely to Hepburn, who has never seemed more masculine despite the absence of her usual suits. Hepburn is the aggressor, sizing up the bumbling paleontologist and seeing that beneath his obscuring spectacles and stick-up-his-ass gait, he is indeed Cary Grant. Naturally, she'd like a piece of that, so she begins contriving wilder and wilder reasons to keep David from his wedding to the frigidly, ironically named Miss Swallow (who almost certainly has no idea what her surname references and would be appalled if she did).
The titular Baby of the film is not, as one might have guessed, a human child but a tame leopard Susan's brother sent back from Brazil for their aunt, the same patron considering the donation to David's museum. Before long, the whole world's gone mad, with David and Susan chasing around their leopard; another, far less agreeable one they let out of a zoo truck under the impression it's Baby; and George, Aunt Elizabeth's dog, who steals the brontosaurus bone David has with him and buries it. All the while, Susan's antics attract attention and her lies grow more and more fanciful.
Hepburn and Grant had already co-starred in two films, the even more gender-bending Sylvia Scarlett and the commercially disappointing but critically lauded Holiday, and they had another coming in 1940's The Philadelphia Story. Their familiarity each other makes for intense chemistry, even as both convince the audience they've never met before in this movie. By the same token, Hawks incorporates enough self-reflexivity—Susan using Grant's character name from The Awful Truth to identify him in an elaborate lie to the cops, the use of the star dog Asta for George—that the look of knowing on Susan's face from the moment she sets eyes on David suggests she has some of her actor's awareness. As fast as the film moves, the actors needed that past working relationship to let them feel so believably attracted. It's difficult to describe how the movie builds their romance even as it never flags, something the film itself points out when David says, "It isn't that I don't like you, Susan, because after all, in moments of quiet, I'm strangely drawn toward you. But, well, there haven't been any quiet moments."
Not quite as fast-paced as Hawks' own His Girl Friday, Bringing Up Baby nevertheless feels the most breakneck of the screwballs even as it is also one of the most carefully composed. Obviously, it takes skill to move at this speed, but that wouldn't automatically make it good: think of all those showboating heavy metal guitarists who wow everyone by playing arpeggios over, and over, and over. Like, yeah, great, you can do scales, now when are you gonna be a big boy and write something? Bringing Up Baby doesn't simply assume that by pushing forward it is funny. It relies on a perfect cast of loony characters* to complement Grant and Hepburn, who crucially play their roles with conviction. Hepburn purportedly did not realize the importance of this at first, overselling her lines because she was in a comedy, but Grant, by then a close friend, set her straight**. If Bringing Up Baby is a film where no one is a reasonable, sensible human being, it is also one where everyone in it likes to think himself reasonable and sensible. That, fundamentally, is why it remains one of the most enduringly funny comedies more than 70 years after its release.
*My favorite side player is Walter Catlett's Constable Slocum, a bumbling sheriff so forgetful he finds himself in pleasant chats with those he seeks to arrest, only to snap back to reality and explode in rage at having been "duped" into treating the perps with pleasantries.
**For a far more thorough account of Hepburn's on-set evolution, read this piece by Sheila O'Malley.