To entertain elderly guests, the British Royal Family would have reels of their favourite silent films sent to the Palace/Castle. Often visiting at the latter was Princess Alice of Greece - the mother of the present Queen's husband (Philip, Duke of Edinburgh). Deaf from birth, she was a highly-skilled lip-reader - and could do so in five languages. She would add to the entertainment by deciphering what the actors were actually saying to each other - in one case, during a dramatic clasp with apparent declarations of eternal love, the leading man was in fact telling his co-star that he was going to be evicted from his apartment if he missed that week's rent.
The disastrous preview showing of The Dueling Cavalier in its entirety. Highlights include:
Numerous distracting misplaced sounds, like the rattling of Lina's pearls.
Don enters and flings his cane away in a melodramatic silent-film gesture—resulting in a deafening clatter off-screen. On that note, you can actually hear him break wind during his monologue - making the thing even more hilarious.
Lina keeps lapsing in and out of her Brooklyn accent, and can never quite seem to aim her lines into the microphone. Don somehow manages to be even worse, comically strutting around the set, turning the melodramatics Up to Eleven, and addressing nearly all his dialogue to the camera instead of Lina.
The following, immediately after filming a Big Damn Kiss scene for The Dueling Cavalier:
Lina: "Oh Donny! You couldn't kiss me like that and not mean it just a teensy bit!" Don: Meet the greatest actor in the world! I'd rather kiss a tarantula. Lina: You don't mean that. Don: I don't— Hey Joe, bring me a tarantula!
The entire "Make 'em Laugh" song number. Every. Single. Thing! Including Cosmo's intro.
Cosmo: In the words of that immortal bard Samuel J Snodgrass, as he was about to be led to the guillotine! ...MAKE EM LAUGH!
"Moses Supposes"—again, including the build-up, from the moment Cosmo trots in: his saving Don from having to repeat a ridiculously long passage...by congratulating the coach and "encouraging" him to say another one (guess which one!)—and then troll-facing the man all the way (the punch line being when the coach catches him)!
There's also the greater implication by the scene that this is stuff Don and Cosmo do all the time when they're bored.
The whole scene with the "All I Do is Dream of You" number from Kathy popping out of the cake and realizing that Don was right there (having previously brushed off Don) to Don snarking about her being a serious actress. At the end, when Kathy is trying to brush off Don again:
Don: Now don't go... now that I know where you live (gestures to the cake), I want to see you home.
Just how gleeful Don is upon finding out Kathy's weak spot. After she punctured his pride and shut him down earlier he is hilariously petty in how much he's enjoying her discomfort.
When The Dancing Cavalier is a smash, and Lina starts reminding R.F. of the promises he made to her, which irritates Don, Cosmo and Kathy to no end. When R.F. seems to be caving:
Cosmo: Simpson, I once lent you a cigar. Can I have it back?
R.F. announces all the films at the studio henceforth will be sound pictures.
Cosmo: That means I'm out of a job. At last I can start suffering and write that symphony.
R.F.: You're not out of job, Cosmo, we're putting you in as head of our new music department.
Cosmo: Why, thank you, R.F. At last I can stop suffering and write that symphony.
Cosmo's assessment of Lina: "She can't act, she can't sing and she can't dance. A triple threat."
Basically everything Cosmo Brown says, but particularly his 'Jazz Singer' impression.
When the company first decides to make a Voiced Production. They quickly become enthusiastic about converting their current film project and throwing themselves out into the talkie scene full-force... until Lina opens her mouth:
R.F.: Don, it'll be a sensation! 'Lamont and Lockwood: they talk!' Lina: [With a voice to peel paint] Well of course we talk. Don't everybody?
The climax moment of the film is this, awesome, and Heartwarming all at once. Starting with Cosmo happily dancing along to the Singin' in the Rain number, with Don and R.F. gleefully joining in as they dance over to the rope to raise the curtain and badly sing along as they do so. The final clincher is when, as the audience roars with laughter at the reveal of Kathy behind the curtain, Cosmo runs out on stage and takes over the microphone from her to continue the song while the oblivious Lina is still lip-syncing. One of the best movie endings in history.
The scene where they're trying to get the mike to work. First, they put it in a bush. Then, they put it over her heart. (It catches her heartbeat). Then, they put it on her shoulder, and it finally seems like all is well...
R.F: [Walks into the studio and trips over wire] What's this wire doing here? It's dangerous. [He yanks it, and sends Lina topping over backwards with a startled shriek]
It's all made better by the director's increasingly frantic reactions.
At the end of the famous titular number, a policeman comes up to Don (who has been making a real spectacle of himself, splashing around in the puddles and dancing in the middle of the road) and basically glares him away. Don's sheepish, cheesy little grin is adorableand hilarious.
Just before that, Don interrupts Lina every time she tries to get a word in. We shortly find out why.
After the whole sequence detailing Don's idea for the number "Broadway Melody" — a massive, show-stopping production with huge sets, lavish costuming, and an army of dancers — it cuts back to Don finishing explaining it to R.F. R.F. only replies that "I can't quite visualize it", lampshading that it would probably look much better on film. (Somewhat of an in-joke, as well: this was a frequent comment made by Arthur Freed himself during pitches.)