The scene where Lisa gets Johnny drunk on a combination of scotch and vodka, capped off with Johnny in 'drunk' mode, which is less convincing than the average ten year old pretending to be drunk;
Johnny: I'm tie-erred, eye'm waysted, I luhrff you duurhling.
"Where's my FUCKING MONEY, DENNY!?!"
"YOU'RE NOT MY FUCKING MOTHER!"
"WHAT THE HELL'S WRONG WITH YOU?!"
"STOP GANGING UP ON ME!"
"Chicken, Peter, you're just a little chicken. CHEEP-cheep-cheep-cheep-cheep-cheep-cheeeeEEEeeeeeEEEeeeeauaa."
Even more hilariously brought up later (during the fight scene) with, "You betrayed me, you're not good. You— You're just a chicken! CHEEP-CHEEP-CHEEP-CHEEP-CHEEP!" He also looks stiff as a board when he does the chicken movements.
Speaking of Peter, the fact that, due to his actor's having to leave the project midway and that Wiseau insisted upon filming the infamous "football in tuxes" scene over the actually-important party scenes, the character's last lines are, " That's it, I'm done," is all the more hilarious.
The extreme close up of Mark's newly shaven face, compelled with the dramatic music, as if it were supposed to be a shocking reveal. Greg Sestero called this one of the most embarrassing things he had ever done.
Another great piece of dialogue:
Lisa: He got drunk last night and he hit me.
Michelle: He hit you?!
Lisa: He didn't know what he was doing.
Michelle: Are you okay?
Lisa: Well, I don't want to marry him anymore.
The drama of Johnny's suicide is ruined by a single line
Mark: As far as I'm concerned, you can drop off the earth! That's a promise.
Johnny's facial expression and silly wiggling of the tape when he shows it to Lisa. It's a lot less "I've got the evidence right here, you bitch!" and more like he's mildly smug.
Despite hitting the Despair Event Horizon, Johnny decides to mourn the fact that his fiancé had been cheating on him by...humping her red dress.
Rotten Tomatoes' summary of the movie.
A benevolent, friendly, selfless man who greets everyone with a disarming "Hi" discovers that you can't trust anyone after getting engaged to a manipulative, self-serving siren who seduces his best friend and destroys his life in The Room. Johnny (writer/director Tommy Wiseau) has everything a man could ever want: great friends, a good job, and a gorgeous fiancée named Lisa (Juliette Danielle). But Lisa's innocent act masks the fact that she's looking to bring Johnny down, and her manipulations are tearing Johnny apart. As Lisa informs her cancer-ridden mother, Claudette (Carolyn Minnott), that Johnny hit her (he did not hit her, that is bull$&*t, he did not), Johnny's best friend, Mark (Greg Sestero), finds his resistance to Lisa's seductive charms weakening. Meanwhile, local orphan Denny (Philip Haldiman) looks up to Johnny, and needs the older man's help after the teen rips off a drug dealer. What kind of drugs? It doesn't matter. Then guys play football in tuxedos, because you can play football anywhere.
"It's okay he just wanted some money." "What kind of money?!"