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What do you get if you cross a Norse god-king with an ego the size of a planet, a nervy science boffin with gigantic anger issues, a WW2 super-soldier with a very silly costume and a genius billionaire playboy with flying armour? Arguments, obviously.

With great power comes great banter in writer/director Joss Whedon’s blockbuster multiplier, which isn’t the best superhero movie ever – but might well be the funniest.

Avengers Assemble is a power-play that’s unprecedented in Hollywood history: launching three different $100m franchises (four if you count the 2008 reboot) to construct one super-mega-franchise. And it has to be said, handing it to a 47-year-old fanboy whose single previous feature film ( Serenity ) couldn’t even scrape back its budget at the worldwide box office was a massive dice-roll.

Iron Man (Robert Downey Jr.), Hulk (Mark Ruffalo), Thor (Chris Hemsworth) and Captain America (Chris Evans) on the big screen together was always going to be a massive kick.

But could any screen be big enough for all them? Would Avengers Assemble look like four bodybuilders in an elevator? The suit, the smash, the hammer, the shield... Like X-Men: The Last Stand , Spider-Man 3 and Iron Man 2 , there was a real danger of heroverload.

Sure enough, it takes Whedon a while to assemble his Avengers, as S.H.I.E.L.D. agents Nick Fury (Samuel L. Jackson), Black Widow (Scarlett Johansson) and Phil Coulson (Gregg Clark) recruit the superheroes to save Earth after Loki (Tom Hiddleston), Thor’s power-mad brother, returns to enslave mankind with an all-powerful cosmic cube.

“What does he want me to do, swallow it?” asks Ruffalo’s Dr. Bruce Banner. And from there, the zingers keep coming.

With X-Men and X2 , Bryan Singer showed how you could disguise a compelling ensemble drama as a superhero actioner. This is exactly Whedon’s speciality – as well as creating cult TV phenomenon Buffy , he worked on the scripts for X-Men and Toy Story – and his screenplay drags together a group of characters with pretty much zero interest in each other.

How? By locking big egos in small rooms and letting the funnies fly. Lifting the movie’s pace every time he steps on screen in the first half, RDJ’s Tony Stark catalyses the Avengers with machine-gun wit.

He gives both barrels to Thor, whether it’s his cape (“Doth mother know you weareth her drapes?”), his kingly lingo (“Shakespeare In The Park”) or the fact he hasn’t had a haircut since his own movie came out (“No hard feelings, Point Break”).

Arguably the least interesting and most poorly dressed of the Avengers, Cap duly suffers in his first brawl with Loki and finds his 1940s brain drubbed like a speed-ball by the sarcastic Stark.

Despite being mere mortals, Jeremy Renner’s laser-sighted archer Hawkeye and Scarlett Johansson’s gymnastic spy Black Widow find key roles in the story. (And, just so you know, the coolest cameo in the movie doesn’t belong to Stan Lee.)

Despite Scar-Jo’s seam-straining catsuit and S.H.I.E.L.D. eye-candy Maria Hill ( How I Met Your Mother ’s Cobie Smulders, a Wonder Woman contender), there’s no sex factor in this superhero sausage-fest.

Instead, maybe the most interesting frisson sparks between fellow brainiacs Tony Stark and Dr. Banner. Third time’s the charm: Mark Ruffalo’s hand-rubbing performance as Bruce feels definitive and Whedon, in a few short scenes, captures a far more dangerous relationship between Banner and “the other guy” (as he dubs his alter ego) than we’ve seen in two previous big-screen Hulks.

One of the problems was that Mr Hyde never really looked like Dr Jekyll. But with Ruffalo’s features used to construct the CG monster’s face, a much more humanised Hulk emerges as the movie’s unlikely stand-out. Once the green giant bursts free, he grabs hold of the movie and yanks it out of Downey Jr.’s hands. The best moments and the biggest laughs belong to Hulk’s smash-happy personality – and only one of them is blown by the trailer.

Unlike Banner, bigness doesn’t come naturally for Whedon. So it’s no surprise that much of Avengers Assemble involves people talking in rooms. He hired Irish cinematographer Seamus McGarvey ( We Need To Talk About Kevin , Atonement , High Fidelity ) for that, but he’s also surrounded himself with a crew of slam-bang assistant directors who’ve worked on everything from The Bourne Ultimatum to Tarantino’s upcoming Django Unchained .

Behind the camera, Whedon’s dream-team help him put his money (more than $200m of it) where his mouth is: after an airship siege high in the clouds, the movie surges towards an exciting, epic extended finale of city-smashing carnage that stretches for a Transformers level of mass destruction.

As Loki’s army pours in from another dimension, one showboating unbroken action shot swoops through the battle to track each Avenger kicking ass against gigantic flying robo-fish and alien warriors riding space chariots.

Perhaps inevitably, there’s never quite enough real drama or danger for our effectively invincible protagonists. But this 142-minute romp between gods, monsters, men and supermen packs so much crowd-pleasing colour and humour that it’s impossible not to walk out grinning.