Requirements for secret ending.
Secret Ending Requirements
After act one, you should have three progress bars in the in game menu, the middle one represents your affinity or friendship levels, aka how much Silverhand likes you.
If it's over 70 by the time of the final mission then you unlock the option for a hidden ending. When you are given the option to select how you want to tackle your final choice, wait a few minutes and it should automatically trigger.
Notes: The titles of the progress bars will change upon completion of the main story for example, straight after act one the title for affinity should be "Play it Again" whilst at endgame would be "You Only Live Once".