The Use of Silence.

Silence is the relative or total lack of audible sound. By analogy, the word silence may also refer to any absence of communication, even in media other than speech. Silence is also used as total communication, in reference to non verbal communication and spiritual connection. Silence is also referred to no sounds uttered by anybody in a room and or area. Silence is a very important factor in many cultural spectacles, as in rituals.

Gestures:

Placing the index finger in front of closed lips is the most widely recognized gesture of silence. The gesture can be used to demand silence without raising one's own voice. The rose, sometimes depicted clasped by or on top of closed lips, is another well-recognized symbol of silence stemming from various mythologies. – This gesture is a possibility to use on the movie poster as it comes across as quite innocent as well as secretive.

Music inherently depends on silence in some form or another to distinguish other periods of sound and allow dynamics, melodies and rhythms to have greater impact. For example, most music scores feature rests denoting periods of silence.

Source: http://en.wikipedia.org/wiki/Silence

Silence in movies creates a lot of suspense and puts the audience on edge. It contributes to the effect of having no idea where you are and who you are with. The use of silence also ties into the theme of secrecy as you’re not engaging with others.