The Command To Look principle states that certain shapes draw the attention and exert a command from the audience to look at them- drawing the eye in so that the person focuses their attention on them, regardless of the actual content of the medium. Four basic shapes are noted as being ideal for this- (I don't necessarily agree with the analysis given)
Diagonal- moving swiftly with determination
S-Curve- slithering motion or body curves
Triangle- blade- threat of sharpness
Dominant mass- block, trapezoid, rectangle- block in path.
Once attention is drawn, the audience may then be predisposed to at least glance at the contents of the medium (of course, the medium may be designed to convey subtler message than its immediate appearance would suggest).
To further draw interest, it's suggested that the content at least touch on the themes of Sex, Sentiment and/or Wonder- ideally conveying some of the depicted experience to the audience- if only vicariously.
The above mentioned shapes can also be used to not only attract the eye but also to direct it to desired areas of the medium.