Doctor Who's Twelfth Doctor plays a WHO Doctor who has a major role in the film's last third. He's seriously credited as "WHO Doctor", and he's now in Doctor Who. This was two months before being revealed to the public as the next Doctor.
Brad Pitt, the film's producer and star, was most intrigued by the book's geopolitical aspects (what with his partner being a UN Goodwill Ambassador and all), and his production company Plan B, together with Paramount, spent $1 million on the film rights. However, it soon became clear that much of the geopolitics that Pitt was interested in would have to be dropped if they wanted the story to come together on screen. Furthermore, Pitt's production company, Plan B, had never taken on a project of this size, its experience limited to eclectic, low-budget dramas; their biggest film before this were the Julia Roberts rom-com Eat, Pray, Love and the superhero Decon-Recon SwitchKick-Ass.
The real problems started with director Marc Forster, Pitt's personal choice to direct the film — and a man whose whose background (not unlike Plan B) was in making smaller, dramatic films like Finding Neverland and Monsters Ball. His only experience making big-budget tentpole films was theBond film Quantum of Solace, known as the Bond movie that few people liked. It was hoped that he would be able to focus on story and characters while his crew could guide him on action and effects, but not only was he unable to bring his usual team with him, the lack of a strong leader at the head of the project produced a muddled vision for what the film would be like. As late as three weeks before shooting was to begin in June 2011, Forster hadn't even decided yet on what the zombies would look like or how they would behave.
Forster and screenwriter J. Michael Straczynski clashed throughout the writing process. Forster wanted to focus on the action, which Straczynski felt detracted from the story's main themes; he was more interested in remaining faithful to the book, focusing on the characters and the global reaction to the Zombie Apocalypse. Straczynski was eventually fired and replaced with Michael Carnahan (writer of The Kingdom and Lions for Lambs), who made the film an action-adventure focused on a UN field specialist named Gerry Lane, dropping the book's first-person accounts. It was at this point that Pitt was cast as Gerry.
And then production began. From the start, it was clear that Pitt, Plan B, and Forster were in way over their heads. Shooting in Malta for the Jerusalem scenes was a nightmare, with two film crews working side-by-side, hundreds of extras, and all sorts of minor costs dealing the budget a Death of a Thousand Cuts. One day, shooting had to be delayed for several hours because the caterer hadn't prepared enough food. When work in Malta finished, the wrap-up crew found a stack of purchase orders related to the cast and extras that had been casually tossed into a desk drawer and forgotten; the amount totaled in the millions of dollars. And all the while, the script still wasn't finished, with work still being done on the third act.
Things got no better when production moved to Glasgow for the Philadelphia scenes. Forster began to fight with both Pitt and the head of the SFX team; the latter was dismissed after principal filming ended. Cinematographer Richard Richardson asked more than once to leave the project, and struggled to keep the crew under control, often antagonizing them in the process. Furthermore, Pitt's schedule conflicted with his commitment to starring in Killing Them Softly, and he also took time off to spend time with his family, pushing production back even further.
During shooting in Budapest in October for the climax in Russia, the crew found out the hard way that their 85 "prop" assault rifles were in fact fully-functional weapons when a Hungarian anti-terrorism unit raided their warehouse and seized the guns. Furthermore, Paramount, after seeing how out-of-control production had gotten in Malta, ordered a scaling back of the budget, forcing the production to scrap a number of scenes. Members of the production criticized the third act as "Rambo vs. zombies", losing the character-driven drama of the rest of the film, and production wrapped with the knowledge that rewrites and reshoots were inevitable.
In June 2012, Paramount ordered, depending on the source, anywhere from five to seven weeks of reshoots totaling forty minutes' worth of the film. They also hired Damon Lindelof to do a third-act rewrite; he later brought in his old LOST buddy Drew Goddard to help him give the script a thorough overhaul after determining that it had much deeper problems. A climatic twelve-minute battle sequence (that they already filmed, at great expense) was dropped entirely, and Peter Capaldi was given a major role. This pushed the film's release from December of that year to June 2013. By this point, the budget had ballooned to anywhere from $170 to $250 million depending on who you ask, and the filmmakers had only 72 minutes' worth of largely incoherent footage to show to the studio.
During reshoots, Forster and Pitt reportedly weren't on speaking terms — Forster's notes for Pitt had to be relayed through an intermediary.
But despite all that, the film was a success. It brought in more than $500 million at the box office, and received generally favorable reviews from critics.
What Could Have Been: As noted above, the original ending for the film was very different, and reportedly involved Pitt's character being conscripted onto a Russian anti-zombie squad before finally escaping back to the US.