Mitzvah 389) That the priests may not occupy themselves with the service of the Levites nor the Levites with that of the priests
“But they may not draw near to the holy vessels and the altar!” (Bamidbar 18:3)
It is a negative commandment that the Levites may not occupy themselves with the service of the priests, nor the priests with that of the Levites. Each one must perform the service specified for him, as it says “Each man according to his service and his load!”. The prohibition is in our Parshah regarding the Levites as it says “But they may not draw near to the holy vessels and the altar!”, and after this verse it says “And they shall not die they and you!”, and this verse comes to teach us that just like the Levites are prohibited from performing the service of the priests, so too the priests are prohibited from the service of the Levites. Chazal said that Rabbi Yehoshua ben Chananyah once asked Rabbi Yochanan ben Gudgedah if he could help him to close the doors of the temple, and he answered him “Turn back! You already deserve the death penalty. You are one of the singers and I am one of those responsible for the doors, and a singer who moves the doors deserves the death penalty.” We see from here that even if a Levite performs a service in the temple which is not the one designated for him, he deserves death by the hands of heaven. Similarly, the priests are prohibited from coming close to the service of the Levites. However, if the priests transgress this prohibition, they do not receive the death penalty, only lashes.
Amongst the roots of this commandment are that the service of these two groups-the priests and Levites-is a precious, holy service, so the work needs to be well protected from despair, laziness and forgetfulness. There is no doubt that mistakes are far more common in work which is the responsibility of two or more people than in work which is the responsibility of one person by himself, as many times they will rely on each other and the work will not be done. This is a thing which is obvious to all people.
This commandment applies when the temple was standing, to the priests and Levites. A Levite who performs the service of a priest or even his fellow Levite, deserves death by the hand of heaven. A priest who transgresses this prohibition receives lashes. It would appear that a priest who helps another priest in his service also deserves the death penalty.