Let's say you have a portrait photo of somebody looking straight in to the camera and you, in retrospect, want to change the gaze so that the person looks, say, to the upper right instead. How would one go about doing this most efficiently in software like Photoshop?
I've done similar things when dealing with bad flash reflections on impromptu group shots. I grabbed the eyes of another family member and pasted them in place. When viewed at 100%, it looks a little odd.
This method really depends on how much of the image you're changing. If the eyes make up a significant part of the scene, it's going to look processed. If this is a full-length shot, then you may be able to get away with it.
A little noise added to the edit will help smooth out the "fake" look, too.
I don't think there is a simple answer to this. There are a few ways of going about it. I would recommend first cutting out the colored part of the eye, leaving only the white (letting photoshop fill in the middle with the clone tool). Then pasting the colored part of the eye on top and moving it to where you want it to be.