An official preview for "Attack on Titan" season 2 was attached to the live-action movie adaptation, and as reported by GeekSnack, an avid film viewer recorded footage of the preview and uploaded the video online, complete with English captions.

The preview footage showcases Squad Leader Hanji Zoe questioning Pastor Nick regarding the Titan stuck inside Wall Sina, the innermost wall protecting the last human civilization from the roaming Titans outside. Pastor Nick immediately warns the people to cover the Titan's face before sunlight reaches it.

Season 1 revealed that the Titans may be getting nourishment and power from sunlight. which means this Titan within the wall may awaken if it basks in direct sunlight for some time. 

The bigger question, however, is why a Titan would even be inside the wall and if there are many more Titans trapped and asleep within the walls protecting the city. Zoe suspects that Pastor Nick knows the truth and has been keeping the information secret for some time. She threatens to kill him by dropping him over the wall if he won't start talking.

However, Pastor Nick explains that "they" did not keep the truth out of malice. This indicates that there are others who know about the Titans in the wall and that there may be a whole conspiracy that has not been unveiled to the rest of the public and the Scout Legion at this point.

After all, season 1 revealed that regular humans could transform into Titans, and the finale episode showcased the plan of using the people within the city as Titan soldiers to combat the wild Titans coming from beyond the walls. It is possible that even more secrets have yet to be uncovered in season 2.

"Attack on Titan" season 2 has no official release date, although it is expected to release in 2016.