He is a young British graphic designer who always wanted to work in the film industry, but even in his wildest dreams Henry Hobson never imagined making his directorial debut with his boyhood hero – Arnold Schwarzenegger.

Maggie, a small budget, independent thriller about a viral pandemic, was an unlikely choice for Schwarzenegger. The film, which will be released in UK cinemas this month, was written by a first-time screenwriter and directed by Hobson, who has spent the last 15 years designing title and credit sequences for other people’s films. It had a budget of $1.4m (£900,000) the director said.

Yet Schwarzenegger came on board as both actor and producer after reading the script, playing against type as a protective father who fights to save his adolescent daughter when she becomes infected with a walking-dead virus. It is both a zombie thriller and a human story about the impact of illness.

Hobson, 34, born in Salisbury, Wiltshire, has no film connections in his family. His father is a university lecturer, his mother a radiographer. He could barely believe it as he watched the footage produced during the film’s 25-day shoot. Schwarzenegger is in almost every scene.

“[I thought] this isn’t quite normal. This is someone I’ve grown up with seeing on screen … a pinch-yourself moment,” he said.

Schwarzenegger became intrigued by the script – written by John Scott, a Nasa engineer – after it appeared on the industry’s “black list”, for the best unproduced screenplays.

“When I read it, I knew I had to do it,” the actor recalls in the production notes. “It is more vulnerable than any role I have played, more real, more emotional. You’re used to seeing me play the ubermensch, the action hero bullets can’t seem to hit.

“In Maggie, I am the everyman … dealing with the most basic concerns – protecting his family.”

After training at the Royal College of Art – inspired by another former student, Sir Ridley Scott, who went on to become an Oscar-winning director – Hobson designed the title, end and credit sequences for films such as Robin Hood and Sherlock Holmes.

There is a “community of ardent graphic design fans”, but audiences are generally more interested in their popcorn or in grabbing their coats when the credits roll: “You’ll see the film, watch your work come on – and then you see all these faces turning [away from the screen],” Hobson said.

He used his skills as a visual artist to woo Schwarzenegger, producing storyboard images that were so intricate and extensive, they spanned some 200 pages. They detailed “my thoughts behind each scene”, he said. “[Schwarzenegger had] never seen that before … It put him in my headspace.”

Hobson produced similar images for the rest of his cast, including Abigail Breslin, Oscar-nominated for Little Miss Sunshine, and Joely Richardson, whose recent films include Endless Love.

Despite taking a break from movies to become California’s governor from 2003 to 2011, his popularity remains high. Having struggled to interest financiers, Maggie’s budget was found as soon as Schwarzenegger was attached to the project.

“He was such a dedicated actor,” said Hobson. “I felt I was able to get an untapped gem with Arnold. He’s such an amiable personality. He puts everyone at ease.”

The film will be released on 24 July. It premiered at the Tribeca film festival in April to mixed reviews.

Rupert Preston, managing director of Vertigo, the film’s UK distributor, said: “For a first-time director on a fairly low-budget, very independent movie, to attach one of the biggest stars in the world is extraordinary.”