iphone 6: Making some apps not need passcode
With the exception of the Clock, Camera, and Calculator apps (which can be opened in Action Center (if you have it enabled in the lock screen)), there is no way to open any app without a passcode.

There is limited functionality with push notifications (but only on certain conditions, like, say, a tweet pops up in the lock screen and you can either retweet or favourite the tweet) and in Notification Center (if enabled in the lock screen), but that depends on whether or not the app has support for either one.