In the first few pesukim of Parshas Tzav, the Torah thrice discusses the fire on the mizbeach, using similar language each time, indicating that there were multiple piles of wood burning simultaneously (see Rashi). However, only in the final instance is the subject of the phrase and the location of the fire stated explicitly, when the pasuk tells us, "A continuous fire shall burn upon the alter" (Vayikra 6:6). On the other occasions the pasuk is a bit vague when it states, "and the fire of the alter shall burn in it" (Vayikra 6:2, 6:6). Presumably the pronoun "it" is referring to the mizbeach itself, but the absence of a specific antecedent, particularly in the latter instance which comes at the beginning of a pasuk, is awkward and creates the potential for an alternative interpretation.
Continuing with the theme of the intervening pesukim which instruct the Kohen about the proper procedure for removing the previous day's ashes from the mizbeach, the Kli Yakar and the Chasam Sofer suggest that the fire mentioned here is not only describing the literal fire of the mizbeach but also alluding to the passion for spirituality that should burn "within him," that is, inside of every Kohen and indeed every Jew. The Imrei Emes (Likkutei Yehudah, Parshas Tzav) echoes this idea but stresses that one's personal excitement and enthusiasm for mitzvos should not be discernable externally but rather remain hidden, burning "within him." He adds in the name of the Kotzker Rebbe, that most of the letters in the Sefer Torah are written in a uniform font size, but the letter "mem" from the word "mokdah al hamizbeach" - "burns on the alter" (Vayikra 6:2) is written smaller than usual, perhaps to indicate that the fire and fervor for Divine connection should be inconspicuous and understated.
The Beis Yisroel marshals support for this notion from the Gemara (Sukkah 28a), which defines the greatness of Hillel the Elder by virtue of the qualities and piety of his students. According to the Gemara, the most exceptional student of Hillel the Elder was Yonasan ben Uzziel and the least impressive was Rabban Yochanan ben Zakkai. Regarding Rabban Yochanan ben Zakkai, the Gemara attests that he successfully explored and mastered every area and discipline of Torah, ranging from the "great matters" to the "small matters." About Yonasan ben Uzziel, the Gemara relates "that when he sat and was engaged in Torah study the sanctity that he generated was so intense that any bird that flew overhead was immediately incinerated."
Why does the Gemara insist on painting a vicarious portrait of Hillel the Elder by detailing the accomplishments of his students. Undoubtably, the exploits of Hillel the Elder's talmidim are a critical piece of his contribution and legacy, but didn't he have his own unique achievements and abilities? The Sfas Emes explains that the Gemara was forced to provide a second-hand account of Hillel the Elder's holiness and impact because his own activities were a mystery even to his closest disciples. Hillel the Elder kept his spiritual pursuits private such that the measure of his knowledge was unknown to others and the intensity of his religious enthusiasm was not apparent. Only by reflecting upon the nature of his students is it possible to get even a glimpse of his persona. But perhaps the tacit testament and true depiction of Hillel the Elder's towering stature was his concealed spiritual identity.
Similarly, the pasuk states in Parshas Behaaloscha with regards to the lighting of the menorah, "And Ahron did so, he lit the lamps toward the face of the menorah, as Hashem had commanded Moshe" (Bamidbar 8:3), to which Rashi comments, "in order to tell the praise of Ahron that he did not deviate." Many meforshim are bothered by the need to recognize and compliment Ahron for his faithful conduct. Why is his dependable trustworthiness somehow surprising? The Bnei Yissaschar (Igra De'kalla, Parshas Behaaloscha) answers that the Torah was noting that while Ahron was presumably ecstatic and brimming with eagerness in anticipation of lighting the menorah, he remained composed and controlled. Other tzaddikim, might have gotten caught up in the heightened emotions of the moment and spilled the oil or toppled the menorah in their anxious effort to do everything correctly. But not Ahron. Ahron was undoubtably emotionally engaged in the mitzvah but his feelings were internalized and his comportment disciplined and unflappable.
Nadav and Avihu's precise sin is hard to pinpoint, the Torah only gives us a cryptic clue when it states, "and they brought before Hashem a foreign fire which He had not commanded them" (Vaykira 10:1). The ambiguous written record gives Chazal wide latitude to speculate. According to the Gemara (Sanhedrin 52a), Nadav and Avihu were punished because they were once walking behind Moshe and Ahron while irreverently murmuring to each other, "When will these two old men die so that you and I can lead the generation." Rashi cites a different opinion from the Medrash, that Nadav and Avihu died because they entered the Sanctuary while intoxicated, which explains why this episode is followed by the prohibition, "Drink no wine or other intoxicant, you or your sons, when you enter the Ohel Moed, that you may not die" (Vayikra 10:9).
The Shem Mishmuel (Parshas Korach) weaves these two versions of the story together and submits that Nadav and Avihu were frustrated by Moshe and Ahron's lack of visible enthusiasm when performing mitzvos. They perceived Moshe and Ahron's dry and stoic nature as robotic and outdated. In their estimation, the passionate pursuit of dveikus needed to be exhibited and displayed externally in order to be dynamic, vibrant, and compelling. For this reason, they entered the Mikdash while intoxicated, because "When wine enters secrets emerge" (Sanhedrin 38a), and their aspiration was to spawn a movement of liberated excitement for mitzvos which had heretofore been systematically restricted and subdued. However, their public parade of emotion and passion is labeled by the Torah as a "foreign fire" because authentic spirituality is inherently reticent and contemplative and not able to be expressed in physical or worldly terms. Therefore, any unprescribed demonstration is, by definition, shallow and superficial, and incompatible with a genuine quest for ruchniyus.
To be sure, holy individuals sometimes move rhythmically with the mitzvos, as the Gemara (Berachos 31a) relates, that when Rabbi Akiva prayed, he would unwittingly drift from one corner of the room to the other. However, the Noam Elimelech (Parshas Kedoshim) cautions onlookers, that any deliberate attempt to mimic or adopt these practices artificially is misplaced and ill-advised. Emphasizing the importance of a random gesticulations, even when accompanying the performance of a mitzvah and intending to express a sincere pursuit of closeness, is a distortion, for it changes the focus from spirituality to externality and transforms an otherwise holy fire into one that is strange and forced. Even an earnest attempt to create a more animated spiritual experience for the admirable and altruistic purpose of increasing and inspiring interest and engagement, can easily slide into religious exhibitionism and virtue signaling, particularly in a society which promotes and celebrates performative behavior.
But alas this method is demanding and hard. For this reason, the Torah introduces the mitzvah to offer the korban olah with the word "tzav" - "command" (Vayikra 6:2), which implies an extra dose of urging and encouragement (Rashi). Why does the mitzvah to bring the korban olah receive more attention and advocacy than any other mitzvah? The Chiddushei Harim explains that perhaps it is because every other mitzvah requires some form of action or speech. All other korbanos were either eaten by the owner, the kohen, or both. Only the korban olah was burned completely on the mizbeach, and therefore aside from initiating the actual sacrificial procedure, nothing was done by the owner of the korban. Without any external activity to focus on, the mitzvah becomes a spontaneous internal exercise, which can be significantly more difficult than operating from the platform of a prescribed deed and course of action. In recognition of this reality, the Torah lends an extra word of support to the korban olah.
We also need chizzuk in this regard. Rav Naftali of Ropshitz (Zera Kodesh, Shoftim) writes, that in order to distract us from real spiritual work the yetzer hara often presents us with an easier albeit less ambitious option. In response to this strategy, the Torah warns us "you shall not take bribes" (Devarim 16:19), because in this scenario the smaller goal functions as a bribe to satisfy our spiritual cravings and abandon our true objective. Let us not be enticed and appeased by the low hanging fruit of religious exhibitions, accessories, and gesticulations, but rather engage in the labor of building an elaborate internal spiritual world where a personal and private relationship with Hashem can develop and thrive.