In the Chabad machzor for Rosh HaShanah, right before the section which begins with the word "HaMelech" (i.e., before Yishtabach and Barchu in Shacharis) the following story appears in small print:

Once, as the holy rabbi, Reb Aharon of Karlin (one of the great disciples of the Maggid of Mezritch), was about to recite [the prayer beginning with the word] "Hamelech" -- he fainted. Later, when he was asked what had happened, he replied that he had been meditating on the words in the Talmud [which were spoken by Vespasian, but which Reb Aharon of Karlin understood metaphorically as spoken by G-d]: "If I am the King, why did you not present yourself before me until now?" Now, what answer could we possibly give?

On Rosh HaShanah, maybe not everyone spends the entire period in the right frame of mind. But if one stops to think for a moment that this is the Day of Judgment, HaShem is reviewing all his deeds from the past year, and one might be on the debit side, then he may think he is in for some punishment. When one gets into that frame of mind, and seriously thinks of all of the negative thoughts and deeds he's done in the past year, then what could happen? His heart can begin to pound from fear of the possibility of punishment that might come to him. One feels like a child who is called to the principal's office, waiting his turn while the principal is on the phone, and he doesn't know what is going to happen or what is going to be said. That fright or terror can actually be felt; little children can get stomach aches and headaches from this fear. There is a feeling of fear that is literally felt in the body. Similarly, the fear of HaShem should not be the kind of fear that is abstract and purely intellectual -- it should be a fear that you actually feel in your heart in a physical way.

However, fear does not only have to mean fear of punishment. Those of you who have studied mussar before you started studying Chassidus, may know that there is a difference in emphasis between the two schools. Both schools, Chassidus Chabad and mussar, have the identical goal -- to inculcate people with love for HaShem, with a will to fulfill the mitzvos, and to go on the right path. However, there is a difference in the means and approach utilized by the two schools.

The approach of mussar emphasizes the dire consequences of a life that does not accord with the Torah; Chassidus portrays the other side of the coin -- the good, the light, the beauty of a life that does accord with Torah.

The Rebbe says a person should not think of fear in the sense of the fear of punishment. Jewish thought does not view G-d as vengeful or spiteful, like someone who is always out to get you with a big stick, looking to catch you if you slip. The Rebbe explains that fear -- yirah -- has many different degrees. Two of these are expounded in chassidic teachings at great length. They are called yereh boshes, and yiras haromemus. Yereh boshes means a bashful fear. When you come to appreciate how HaShem is kind to the undeserving (which you learn by studying Chassidus), you may start to feel ashamed that you do not behave as He expects you to. You may think of where you are and how you sometimes resent doing the things HaShem is asking, which are so small compared to that which He gives to you. If you think about this seriously, you come to the realization that you really are so far from where you should be. You come to a feeling of embarrassment: "Oy, look at the way I am, in relation to HaShem, and compared to the way I should be." This shame is called yereh boshes.

The other type of fear is called yiras haromemus, and that is more like a feeling of awe, of seeing how great HaShem is and how small I am. Both of them are variations of the concept of fear, but they are not the same thing as saying, "I'm afraid I'm going to be smacked." What kind of fear Reb Aharon of Karlin experienced I do not know, but it was certainly not fear of punishment -- he was a righteous tzaddik who certainly did not succumb to his Evil Inclination. Why then did the Alter Rebbe include this story in the machzor? To indicate to us what type of fear is expected of us. Not fear of punishment, but yereh boshes, and yiras haromemus.