How Comic Con can make or break a movie: From Batman vs Superman to Star Wars: Episode VII

This weekend, 130,000 fanboys and girls will descend on San Diego for the 46th annual Comic-Con. In recent years, the sprawl of film screenings, question-and-answer sessions, comics workshops, merchandise stalls and more has spilled from the 2.6 million sq ft of the Convention Centre into venues across the city.

But for many attendees, there is only one location that matters: the hallowed Hall H, capacity 6,500, where the Hollywood studios present their blockbuster wares to fans who will queue for up to 24 hours for the privilege of seeing a major movie presentation. The online buzz created by the Hall H crowd – call it "The Comic-Con Effect" – can give a big movie the momentum to become a mega-hit.By contrast, a negative reaction in San Diego can send a studio scrambling to salvage its next tent-pole's reputation. For one long weekend a year, the Hollywood marketing machine cedes its influence to the geek citizenry. "I don't feel l have individual power during Comic-Con," says Alex Billington, founder of the film blog FirstShowing, "but we acquire collective power as a geek group."

In 2008, Marvel was an untested movie studio and Iron Man, a superhero known only to comic-book aficionados. Jon Favreau, the director of the film version starring Robert Downey Jnr, later attributed its vast success to the word-of-mouth generated when he showed footage at Comic-Con that year. Iron Man was the first film in a Marvel portfolio that has since grossed more than £5.5bn.

Mad Max: Fury Road flew under the radar until its trailer debuted at Comic-Con in 2014, sparking excitement across the internet that translated into more than £225m at the box office in 2015. "The buzz that comes from being there with thousands of people in San Diego, which goes around the world on Twitter – a TV ad can't compare to that," says Peter Sciretta, the editor-in-chief of Slashfilm.com.

A Comic-Con buzz can be deceptive, however. In 2009, the trailer for supernatural Western Jonah Hex met with widespread approval in Hall H; in cinemas, the film turned out to be a turkey. Edgar Wright's quirky comic book adaptation Scott Pilgrim vs The World was the darling of the 2010 convention, but a dud at the box office, making back just £20m of its £38m budget.

Marvel Studios will be absent from this year's convention, which opens tonight and ends on Sunday. But Hall H will be packed for Lucasfilm's Star Wars presentation on Friday night and a Warner Brothers event the following day, at which fans expect to see new footage from Batman vs Superman: Dawn of Justice, which won't be released until March 2016.

That film's director, Zack Snyder, also made the comic book adaptations 300, Watchmen and Man of Steel, and is known as a master of the Comic-Con reveal. "The way [Snyder] teases and builds anticipation in an audience – he knows just how to play them," says Billington. "He's so good at presenting that you can't help but be excited about his films."

Both Batman vs Superman and Star Wars: Episode VII (due out in December) seem too big to fail, but other titles may find their chances boom or bust on the Comic-Con effect. The forthcoming Deadpool from 20th Century Fox has a wisecracking antihero beloved by comic- book readers; its Comic-Con reception on Saturday will signal whether it's another Iron Man or the next Jonah Hex. Fans also expect to see early footage from Warcraft, British director Duncan Jones's film version of the hugely popular online fantasy game, World of Warcraft; their first impressions will be crucial.

What Hall H really wants is not just a trailer or a clip, but an event. When Tom Hiddleston appeared on stage as the Avengers villain Loki in 2013, Sciretta says: "You could feel the magic in the air. The studios like to think people go to Comic-Con to see their footage, but I think a lot of fans are there to have that kind of experience, the Loki experience. They're in Hall H to witness something special."