It conveys so much emotion, complimenting the facial emotion. Many times you can feel in the hand pose, the strain, the excitement, the weakness, the power, etc. Yes, it contributes to the feeling and not really seeing it. And if the hand pose is off, too neutral or weird, it will distract the audience away from the character's performance.
Another thing to take note is clear readability of the silhouette. It may look good in one camera angle but not the next. Always pose to the camera even if the rig has to break a little to look good.
So, the best way is to practice. I took many pictures of my hand making as much interesting poses as possible and translate it to Maya.
Here is an example.
Fingers' contact to a solid object is also good to practice. It must look believable that there is tension with the contacted surface, and also not too neutral pose.
