Download the Construct 3 Dialogue Template here:
Dialogue Mechanic Explanation
The white, translucent dialogue box, the text and the E symbol all follow the player box using Hierarchies.
All the above objects as well as the player box are initially invisible.
When the player crosses the purple line it triggers the Minotaur to face him and the E (interact) symbol to become visible and flash.
Makes the dialogue box visible
Makes the SpriteFont text visible
Starts the TypeWriter effect of the text
When the typewriter effect is finished:
Make everything invisible again.