Select the 'Aggressive', 'Diplomatic' or 'Defensive' responses and talk to ALL the characters in an HQ. Then save the game, quit, and reload that area and repeat ALL the conversations with different responses. When you do this you will add to your previously earned Attitude points.
Complete the game under Heroic or Super Heroic difficulty setting. When you accomplish this task and then go to the Briefing Console the option to start a new game in Legendary mode with ALL previously unlocked heroes, level and abilities becomes available.
This feature becomes an available option when you complete the game. In New Game+ mode you begin on the Legendary difficulty setting with your characters retaining the levels they had from the completed game.