Desire provides the energy for manifestation while imagination "images" the outcome we seek ... not necessarily a visual image, often it is just a feeling, or a "desire for fulfillment".
Expectation is the tough one. It is the sincere, deeply held belief that what we desire and imagine is there for us and will manifest. The "letting go" that Seth talks about is really what expectation is all about .. the state of grace knowing that we are divine beings enjoying a physical sojourn that we are manifesting for the sheer joy and love of being.
“To “let go” is to trust the spontaneity of your own being, to trust your own energy and power and strength, and to abandon yourself to the energy of your own life. The word “abandon” itself may strike some readers as particularly strong, but each element of nature abandons itself to the life form. So does each atom of your body. To abandon yourself, then, to the power of our own life, is to rely upon the great forces within and yet beyond nature that gave birth to the universe and to you.
“One of the very first steps toward mental, physical, emotional and spiritual health is precisely that kind of abandonment, that kind of acceptance and affirmation.
“This feeling of abandoning oneself to the power and force of one’s own life does not lead to a mental segregation, but instead allows the self to sense the part that it plays in the creative drama of a universe. Such understandings often cannot be verbalized. They are instead perceived or experienced in bursts of pure knowing or sudden comprehension.”