By no means applicable to all, but I believe this is applicable to many.
When you are tense, like, during an interview or emergency situation, your brain fired up at rocket speed to scan all your experience. And in some case your instinct as well – like it tell you to run immediately when you are hunting. (But sometimes, your brain freezes/stalls instead)
When you are relaxed, like during showering or cruising, your brain relaxed and lets your imagination runs wild. This is why a lot of people experienced that “I thought that during the shower!” kind of experience.
Conclusion? Utilise this knowledge about how your brain reacts to help you. If you need to be creative, get relaxed. If you want no error sure-fire safe answer, get tensed!