The use of the imagination is distinctly human. However, imagination can be abused. Fantasy can be the use of imagination not only to avoid action but to avoid feeling. While elements of a feeling can be inferred from a fantasy, usually, the more extensive the fantasy, the less felt is the possible feeling. The pretending of children is different. Only as much pretending is done as to enhance feeling and excitement. During pretending, there is usually interaction with actual others, and there is almost always a great deal of movement. Fantasy, on the other hand, is solitary, and the fantasizing person is usually motionless. Fantasy has often been a refuge for people for whom neither spontaneous action nor spontaneous feeling was reliably safe in childhood.
Unlike fantasy, (and unlike opinion), feeling impels movement and contact. A general guide to regaining satisfaction is to put the energy into the feeling not the fantasy. If feeling is thought of as having the density of a liquid, then fantasy might be thought of as having the density of a gas. Fantasy is a low energy activity, but it tends to expand to fill available mental space. The possible deception is this: a person with a lot of fantasy may believe there is a lot going on, but feeling, for the time being at least, is low. Folk wisdom speaks about the danger of 'talking' about something all the time and not actually doing it. Possibly this is because on some levels the mind and brain do not distinguish between fantasy and action.
Many important fantasies tend to be 'secret', and while many people try to keep feelings secret also, feelings tend to leak out while fantasies do not. Disclosing a fantasy to another person is often awkward, in part because it is often very distant from reality, and also because these fantasies are often organized around aggressive or angry feelings that are vehemently denied otherwise. Fantasy should not be thought undesirable because it contains 'negative feelings' but rather because it helps prevent these feelings from being integrated.
One type of fantasy is concentrating on how things should be rather then on how they are. Yes, imagination has a role in making things better, but fantasy often acts as a non-acceptance that actually undermines creative action.
Imagination and Visualization: Visualization is holding in the mind's eye a shape, a movement, a color, or even a texture. This activates creative sections of the mind and activates the body and the senses. Visualization has an important role in bodywork for this reason. Some knowledge pops into the mind as a 'picture.' This is an important part of intelligence. The 'involuntary' functions of a person are hard to reach by the will, but can be affected by visualizations and 'intentions' in the sense of "suggestions to oneself." Fantasies on the other hand, are usually about a circumstance or situation-- this is the province of the ego.
On a different note, a fantasy offers useful information about what feelings would be if feelings were flowing. This may not be straight-forward because a willed part of fantasy is mixed with a subconscious part. A therapist skilled with emotion may be able to help if fantasies are described.