During a recent interview with the developers of The Last of Us by Gaming Examiner, many details were delved into about the upcoming gem from Sony. Some of the pointed bits of info being the confirmation of a form of multiplayer.

While the main game will not offer co-op, the developers did say that  they have something else in store.
"We are supporting multiplayer with The Last of Us. We’re not ready to talk about the details of how multiplayer will be implemented, however we can say that it is not co-op within the main campaign."

My theory is something akin to Uncharted's online multiplayer mechanics with a focus on team or squad-based battles. Perhaps two factions fighting over limited resources or territory. This could prove to be even more interesting if the biological monsters shown in the debut trailer for the game make an appearance as natural disasters that sweep through the field.

Along with mentioning multiplayer, the interview also took a deeper look at the gameplay set-ups and the way the two main characters develop. One of the methods showing that the relationship between the protagonists is a fluid system that shifts and changes depending on how the player decides to pursue things. Certain items, scenes, or events can trigger dialogues or provide a bit of information about the characters' past. One example being when the Ellie points out a movie poster during the recent gameplay demo. Going further on the idea, Naughty Dog went on to say.

"The relationship between Joel and Ellie begins when Joel has been tasked by his dying friend to smuggle Ellie to another quarantine zone. It starts as a job he’s taken on. As we demonstrated in our behind closed doors demo at E3, aspects of Joel and Ellie’s relationship and their personalities can be revealed by interaction with various elements, such as an old movie poster, throughout the game world. You’ll find out more about Ellie and Joel’s journey as the story and their relationship evolves throughout the game. If you choose to fully explore the game world you’ll be able to dive more deeply into their stories and personality quirks."

With this added emphasis on the relationships between Joel and Ellie and the ensuing cruelty of the new broken world they live in, The Last of Us is shaping up to become one of the most striking games to be released both visually and emotionally.