According to Osho, spontaneity cannot be cultivated; it appears when we stop adding and start removing. Work on yourself negatively: practice NETI NETI - dropping thoughts, memories, desires and roles - until the mind's barriers are empty. Like digging a well, clearing the rocks lets the inner water flow. When nothing blocks your self-nature, spontaneity bursts forth naturally, deep and authentic, not an acted mask.
Stop trying to act natural; gently clear away busy thoughts and labels, and your true self will move on its own.