The smug, annoying student who provokes Frederick to this level is played by Brainy Smurf. Which just figures.
When Frederick stabs himself in the leg with the scalpel, the way he pauses and looks at it sticking out of his leg, then "casually" puts his other leg up to obscure it and act as if nothing is wrong.
At the station mid-embrace:
Elizabeth: Taffeta, darling.
Frederick: Taffeta, sweetheart!
Elizabeth: No, the dress is taffeta. It wrinkles so easily.
At the end of their conversation, having hardly touched Elizabeth, Fredrick settles for blowing her a kiss. She dodges it.
The scene on the front steps of the castle with Frederick, Igor, Inga, and Elizabeth. The entire film crew was laughing so hard they had trouble shooting it.
Frederick: Igor, will you give me a hand with the bags? Igor:Soitenly! You take the blonde and I'll take the one in the toiban! RAWWWR!
On the DVD featurettes, you can see at least half a dozen failed takes of this scene where everyone cracks up. In one, Marty accidentally comes away with a piece of the fur stole hanging out of his mouth.
Beforehand Frederick and Igor greet Elizabeth at the castle:
Elizabeth: Darling!
Frederick: Darling!
Elizabeth: Surprised?
Frederick: Surprised!
Elizabeth: Love me?
Frederick: Love you! Well, why don't we turn in? I'll just pay the driver.
Igor:(to Elizabeth in the same manner as her) Darling!
Gene Wilder's dance after the light explodes and he tries to convince the monster to resume dancing. "Are you trying to make me look like a fool!?"
"AAAIIIIIIIIIIIIIIIII...ain't got no-bohhhoody, and nobody cares for me. Yakka ta ha..."
"Eye-gor!" "Froderick!"
"Call it...a hunch! Dadum-tiss!
The three are planning how to make reanimation a success:
Frederick:(reading his grandfather's work) "The minuteness of the parts formed a great hindrance to my speed, I resolved therefore to make the Creature of gigantic stature." Of course, that would simplify everything.
Inga: In other words, his veins, his feet, his hands, his organs, would all have to be increased in size.
The switch in the lab marked "The Works". The one that Igor is hesitant to throw when they're awakening the monster and kickstarts the special effects that are...well, The Works.
Frankenstein's journal on reanimating dead tissue is titled, simply, "How I Did It".
As he reads the journal on, well, how he did it, it explains how he had his eureka moment when he switched the polarities, implying that it wasn't working at first because he put the batteries in backwards.
The fact that Marty Feldman kept shifting his Igor hump around inspired an ad-libbed joke from Gene Wilder: "Didn't you used to have that on the other side?"
When the trio are preparing to attempt revival, Frederick's shouting to Igor up on the roof to "Tie off the kites and get down here as fast as you can!" When there's no reply, he loudly repeats himself right as Igor steps into shot beside him, seemingly having teleported down, and confused about why Frederick is shouting.
The experiment has seemingly failed. All are having dinner. Frederick is morose.
Igor:(wistful) You know, I'll never forget my old Dad when these things would happen to him, the things he'd say to me.
Frederick: What did he say?
Igor:(angry) "WHAT THE HELL ARE YOU DOING IN THE BATHROOM DAY AND NIGHT?! WHY DON'T YOU GET OUT OF THERE, GIVE SOMEONE ELSE A CHANCE?!" (goes back to eating, wistful again)
In the Italian dubbing the line is slightly changed, but just as hilarious:
Igor: "When Fortune is against you and you haven't reached success, stop fantasizing and go cry on the toilet!"
Hermit:Wait! A toast...to friendship! [smashes his mug against the Creature's too roughly, breaking it] Creature: *rolls his eyes in annoyance and disappointedly drops the handle on the table*
The monster's reaction to him pouring hot soup in his lap and having his thumb lit instead of his cigar.
The charades scene, where Frederick, being throttled, struggles to tell Inga and Igor to sedate the Monster. "SEDAGIVE?!" Even better in that the Monster and Frederick give each other an Aside Glance at that particular guess while The Monster is choking his creator.
Igor's excited "Dirty word! He said a dirty word!"
Frankenstein wants to talk to Igor about the brain. When he invites Igor to sit, Igor sits on the floor first before being offered a stool.
Frederick: Are you saying that I put an abnormal brain into a seven-and-a-half foot long... fifty-four inch wide...(begins strangling Igor)GORILLA?!?Is - that - what - you're - telling - me!?!
Inga: Three syllables, yes- *banging on door interrupts the group*
Frau Blücher (*NEIGH*) finding Frederick and Inga post-coitus, which puts an uncomfortable frame on her message that Frederick's fiancée is arriving shortly. Not only do we see that Frederick has motorized the elevator platform with a button since we last saw it, seemingly expressly to use it as a sex bed with a sky view, but Frau Blücher (*NEIGH*) ends the scene with the acerbic, withering line "I suggest you put on a tie!"
The Running Gag of FRAU BLÜCHER!(Dramatic thunderclap, horses whinny in fear).
Especially Igor just popping out and shouting "Blücher" (*NEIGH*) to make the horses freak out, and Marty Feldman's impish grin.
Frederick tells the others not to let him out of the room while he's there alone with the monster, no matter what. Naturally, the instant the monster wakes up...
Frederick: Let me out, let me out of here, get me the hell out of here. What's the matter with you people?! I was joking! Don't you know a joke when you hear one?! HA-HA-HA-HA! Jesus Christ, get me out of here! Open this goddamn door or I'll kick your rotten heads in! Mommy!
Realizing he's cornered with no help to get out, Frederick thinks fast before abruptly saying "Hello, Handsome!" The monster looks around in confusion, checking for anyone else around before looking back at his creator uncertainly.
Frederick continues to heap on the flattery, emphasizing the Olympian ideal and calling the monster a god. From there, he says that he believes the "monster" is good. Turns into Heartwarming when the monster starts crying from the acceptance and Frederick has clearly realized – genuinely – that his creation is only a scared child in a disproportionate body.
"You haven't even touched your food." "There. (mashes the food with his hands) Now I've touched it. Happy?"
The Creature finally awakes while the three are having dessert:
Igor: What is this?
Frederick: Schwartzwalder Kirschtortenote a traditional black forest cake.
Creature:(offscreen) Mmmmmmm!
Frederick:(to Igor) Oh do you like it? I'm not partial to desserts but this is excellent.
Igor: Who are you talking to?
Frederick: You. You made a yummy sound so I thought you liked the dessert.
Igor: I didn't make a yummy sound. I just asked you what it was.
Inga: It wasn't me.
Frederick: Well now look here if it wasn't you and it wasn't you- (gestures to himself in confusion; the gang all drop their silverware and rush to the lab).
"Now, let us all go to my house for a little spongecake, und a little wine- [The monster accidentally pulls Kemp's wooden arm off]Und shit![Holds and regards his arm for a second, then dramatically points to the door with it] To the lumberyard!"
Prior to all this, as the townsfolk were storming the castle, they were using Kemp as a battering ram to get the doors open.
According to the DVD Commentary, Mel Brooks had to stick a handkerchief in his mouth when filming most of the scenes so he wouldn't ruin them by laughing from behind the camera. He had to do the same for the entire crew.
During Fredrick's train ride to New York there is an old couple complaining about their son's masturbating, with the husband saying the wife should just let him do what he wants. When Fredrick is on the train to Transylvania there is another old couple... who have the EXACT same conversation in German.
Mel Brooks later gave the full story of how he broke it to the studio execs that the movie would be black and white, springing it to them literally on his way out the door after the deal was signed. As he describes it, there was soon a chorus of "thundering Jews" shouting after him that Peru had just gotten color theaters.
The musical
The "He...vas...MY...BOYFRIEND!!!" moment becomes an entire song in the musical.
In The Musical, the horses whinny even when Frau Blücher just mouths her name.
It's pure majesty.