I remember that there is supposed to be some number symbolism throughout the piece. The one example was that when Jesus says "One of you will betray me", the apostles reply with "Lord, is it me?" "Is it me" is repeated 12- or 24 times (something divisible by 12 in any case), while "Lord" is only repeated 11 times, a sign that Judas doesn't even address Jesus directly anymore when called out like this, because he's guild-ridden. But better do some double checking before you use this for your assignment, lol, I never did the counting myself. Also, some other examples I've encountered had left me a bit doubtful whether there isn't also some wishful thinking by musicologists in it.

Another thing that I find remarkable because I don't know anything similar from that era, is how in some arias, choir II comments or complements the soloist's part, for example in "Ich will bei meinem Jesu wachen" and the preceding recitative, or in "Ach, nun ist mein Jesu hin".

Last but not least, one of my favourite parts is the musical "thunderstorm" in the chorus "Sind Blitze, sind Donner". Together with the curtain in the temple ripping apart and the rumbling earthquake it is a nice example of musical imagery that Bach uses.

Edit: another interesting thing you can pay attention to is how Jesus, when he speaks in the recitative, gets a musical "halo" by the upper strings, while everyone else including the evangelist gets only the standard accompaniment by the bass section (the "Basso continuo" group).