Usually, the key to achieving the mood you want is by using the right kind of lines and shapes. Graphical elements can impart a strong emotional feeling, without the viewer even realizing that they're being influenced in that way. You should use that to your advantage when trying to create a certain mood. And to make sure you're not using the wrong kind of shape and undermining what you're trying to say.
Again, this seems kind of simple and obvious, I know. But I can tell you that I've used all these "tricks" myself time after time and they've really helped me out and helped give my work the effect I want.
Vertical lines are great for imparting height. They are great for giving a sense of grandeur and magnificence. They should be used for making things look impressive and awe inspiring. They feel elegant and sophisticated.
Castles always make me think of vertical lines. I wouldn't know how to draw a castle without verticals. Certainly it's an easy way to make them feel imposing and impressive.
Being at the bottom of The Grand Canyon and looking up at the verticals above you would certainly make you appreciate the feeling that verticals can give you.
By contrast, horizontal lines give a feeling of peace and calm. They feel stable and comfortable. Whenever you see someone sitting on a peaceful beach, the horizon creates a strong horizontal line that feels very relaxing and peaceful.
Another variation is the horizontal "S" curve composition. If you have elements creating an "S" shape in the frame, it takes time for the eye to travel along the curved shape, and that can be helpful for making the eye linger on an image.
Franquin used an "S" shaped composition for the last panel of "Le Repaire de la Murene" to make your eye linger over the frame for an extra beat and give a feeling of finality.
Diagonal lines tend to feel unbalanced and unsupported. They usually give a composition a violent or unsettled feeling. They are good for action scenes and scenes of tension, suspense or scariness. Here are some examples of diagonals in animated films where they're employed to convey this type of feeling:
A frame of Gollum in his cave that uses diagonals to give a creepy, unsettling feeling:
These pictures of abandoned places are composed to create diagonals in the frame, which enhances the crazy, off-balance feeling you get from viewing them:
Circular shapes give another type of feeling. Circles are soft looking and feel comfortable, reassuring and un-threatening...maybe because they don't have any edges. Circles can be great for creating soft, comfortable, homey and quaint environments.
The earliest Disney characters were all curves, with no edges or straight lines. I think a big part of their charm and quaint look is because they are all based on circles and that gives them a comfortable, cozy feeling.
Anyway, these are just my first impressions off the top of my head on how I think of these different types of shapes and how I tend to use them. These are not hard and fast rules at all. After all, the Death Star is all circles...and it's pretty threatening, and not at all comforting!
All of these thoughts on shape are totally pliable and open to interpretation.
Lastly, here is a sheet from one of Andrew Loomis's books where he talks about the psychology of each type of shape and line:
If nothing else, just remember that every type of line and shape imparts a certain feeling. Make sure you're choosing the right line or shape for what you're doing, and don't undermine yourself by accidentally making the wrong choice.