Anyone watching last Sunday's episode of Channel 4's Homeland will recall something unusual happened during the commercials.

If you picked the wrong time to make a cuppa, here's what you missed: a trailer for Ridley Scott's new Alien prequel Prometheus, which took up the entire ad break, followed in the second ad slot by audience reaction to that trailer in the form of posts from social media site Twitter. These were read out like congratulation telegrams at a wedding, an appropriate simile for what was an exciting marriage of old and new technologies.

It was also the latest salvo in a war pitting four much-admired movie franchises against each other, which some say is shaping up to be the best summer blockbuster season in years. It's a battle in which social media, the internet and the dark art of viral marketing could be almost as important as the films themselves.

"Prometheus isn't screening to anybody until the day before release, so they're really building it cleverly," says Screen International editor Mike Goodridge. "Nobody really knows what the connection to Alien is, though there's alien imagery in the trailer. But it looks fantastic and I think Fox has a huge movie on its hands there."

Scott's film is released at the start of next month and is the second of the four films out of the blocks. The first, Joss Whedon's Marvel Avengers Assemble, was released in the UK by Disney on April 25 to universal acclaim and a first-day take of £2.5 million. July sees the arrival of The Dark Knight Rises, the latest in Christopher Nolan's acclaimed Batman series, preceded by Columbia Pictures' The Amazing Spider-Man, with British actor Andrew Garfield as the Lycra-clad web slinger and newcomer Marc Webb in the director's chair. Taking his lead from the success of Nolan's Batman "reboot", Webb is promising a darker, moodier take on the comic-book favourite.

"In space, no-one can hear you scream," read the tagline on Ridley Scott's first Alien film. The ker-ching! of box office tills ringing up sales is another matter entirely, and each of the four studios involved will be hoping for a bumper year. With around 40% of box-office take coming from the summer months, and each film looking to recoup nine-figure budgets, there's a lot to play for.

"It is a big, big summer and that's a trend that's been going for a few years," says Helen O'Hara of Empire magazine. "Film-makers have been cutting back on middle-of-the-range productions and focusing on films that are either very expensive or very cheap. Now the summer is very much all about the blockbuster."

It's also very much about viral marketing campaigns like that run by Warner Bros for The Dark Knight Rises. The studio launched a website for Nolan's film last year and on it placed an encrypted sound file. This gave the address of a Twitter account on which the company revealed the first picture of Tom Hardy as the latest villain, Bane. But they did it gradually, building traffic by uploading one pixel for every tweet. In other words, they made the fans work for it.

The studio used the same approach last week when they stepped up the campaign. Two days after Channel 4's Prometheus stunt, a new (third!) trailer for The Dark Knight Rises was uploaded, frame by frame, as fans posted images of dozens of white bat symbols which had been hidden at sites across US cities.

"Will the Dark Knight Rises have tens of millions of dollars thrown at it when it opens? Of course," says Goodridge. "But the studio wants to create a must-see factor and that's where viral marketing comes in. The fan boys are very powerful these days - The distributors know exactly how to manipulate these groups and what to provide them with."

Even before Nolan's film started shooting, in-house teams and outside specialists such as LA-based 42 Entertainment, who worked on the previous Batman film, will have been cooking up viral and "stealth" marketing. Much of this will utilise what's known as the "subdural" approach in which the message is hidden rather than broadcast, with a trail laid down for fans to follow. Put a strong back story in place around a film using these tactics and you have a powerful marketing tool and, it's hoped, an all-conquering brand.

The grand-daddy of internet viral marketing was The Blair Witch Project back in 1999. Marketeers created a stir about the film by setting up a website filled with details about the "real life" killings and supernatural goings on in Burkittsville, Maryland. The strategy was unheard of it at the time. It changed the game forever and the tactic is now de rigeur for producers wanting to milk every buck from the viewing public.

Today the movie experience begins when punters download the first, second and third trailers to their smartphone; it continues when they immerse themselves in the film's complex mythology and expensively constructed websites (Prometheus has one called Weyland Industries), and reaches its climax – or maybe its nadir – when they indulge in arcane Twitter discussions about aliens and superheroes using hashtags like #nolanisgod and #teamironman (yup, they're both out there).

"I think what's really interesting is this trailer one, trailer two, trailer three mentality," says Goodridge. "It's this notion that every few months they'll release a new trailer. It's tantalising. It's building up awareness with media and with audiences, to the point where, when the movie opens, the target audience is in a frenzy of excitement."

BUT while the studios – and increasingly the directors – throw time, talent and resources at complex stealth and viral marketing campaigns, their efficacy in terms of bums on seats remains hard to quantify, says Total Film's Rosie Fletcher.

"Viral marketing is an unknown quantity by its very nature because you're asking people to spread the word and build the buzz, but these aren't people you're paying, they're ordinary people. So nobody has been able to identify exactly what the value of it is and what combinations make up the magic which turns something into a success. But certainly with some movies it seems to have enhanced the buzz."

She feels the same way about Channel 4's Prometheus stunt. "A film like that doesn't need such a hard marketing push anyway because it has all the names in it, Ridley Scott directing it, and of course it's part of the Alien franchise. So while I thought it was cool, I'm not convinced how much difference something like that is necessarily going to make. But the spot was interesting because it's pitching to a certain market – it's less teenage boys and more of a serious adult demographic. It's not the Transformers crowd."

There's even a danger that viral marketing campaigns can backfire. "You can gift people something and they will like it and talk about it, but if you push it too hard they'll think they're being sold to and they'll react the other way," she says. "It's a delicate balance to strike."

It would be gratifying to think the quality has to be there in the first place for the viral marketing campaign to work. That isn't always the case, though. Take exorcism-themed horror film The Devil Inside, predicated on a cache of found footage and acquired by Paramount for less than $1 million. It was roundly panned on release in January but took nearly $35 million in its opening weekend, a feat which knocked Mission: Impossible – Ghost Protocol off the top spot.

How did it do it? With a clever teaser campaign which filmed audience reaction to a test screening held in a church and presided over by a real priest. People thought it was real. When they found out it wasn't, they were still intrigued enough to see if the fiction matched up. It didn't, but by then they had paid the admission price.