Giving a character the expression you want can be a tad difficult at times T.T
Expressions are made up from a combination of the mouth eyes, and brows. Downward pointing brown and angry eyes combined with a smile will make some one look evil/sinister, while the same combination except with an open mouth can show a sort of angered surprise? It all comes down to mixing the right combos to make the right expressions.
The eyes are an incredibly expressive feature. This varies from how wide open the eye is ( surprise or anger), to a narrowed eye ( anger ). The skin under the eye lifts up and makes the bottom of the eye appear almost flat when smiling. Remember different muscles will effect the eye according to what emotion you want to portray, I suggest getting a mirror and having a look at your own eye.
This can be used to easily express surprise, confusion, happiness and sadness. Upward sloping = sadness, surprise downward sloping = anger, surprise, concentration. Horizontal lifted eyebrows = indifference, unimpressed. one sloped up and one sloped down = puzzlement, annoyance.
The mouth is pretty straight forward when showing expression, though a smiling mouth will have different meaning when combined with other different facial expressions.
Just experiment and look and see how other people do draw expressions, also looking at real life helps.