When you are pensive, your eyes stare blankly or dreamily into space and your head tips slightly to one side as though you are absorbed in trying to catch on a thought, ignoring your surroundings.
When you are happy, your eyes crinkle, your lips curve upwards and you may throw a quizzical look with those tiny folds above the eyebrows if you find something absurd - like, "Huh? Hahahhaa...".