Marnie is the first character to fall victim to the fourth Ghostface killing spree.
Marnie appears at the beginning of the movie watching Stab 7 alongside Jenny. She discusses the idiocy of Stab 7 's opening sequence being Stab 6, which refers to the opening of Scream 4 being that of Stab 6 and Stab 7. Jenny claims to hear a noise upstairs and goes up to investigate, leaving Marnie alone downstairs. Marnie thinks Jenny is just trying to play a prank on her.
At that moment, Ghostface calls her and asks her the infamous line, "What's your favorite scary movie?" However, it is revealed that it is only Jenny on the other end, using the Ghostface App on her cell phone to trick Marnie. Jenny hears Marnie cry out and runs downstairs to find her. She assumes that Marnie is trying to trick her and walks around the home looking for her. The real Ghostface calls her, telling her she's in his new movie now. The same movie Marnie was in but her part had gotten cut way back. When Jenny asks how, he insults her before warning her that she'd "better start running." Then Marnie's dead body is thrown through the window and Jenny runs. She is chased and finally killed in the garage.
Marnie is mentioned by Robbie after Olivia Morris dies, saying "Olivia is dead. So are Jenny Randall and Marnie Cooper." In the climax of the movie, Charlie shows Sidney Prescott a video clip of Marnie being stabbed in the stomach by Ghostface.
In the alternate opening, Marnie was originally the main victim. Jenny hears the noise upstairs and goes up to investigate but this time, after pranking Marnie when she comes back downstairs, she sees Marnie lying next to the couch seemingly dead.
After a moment, she smiles and laughs. She was only playing a prank on Jenny to get even, revealing she knew all along that Jenny was up to something. They sit on the couch and agreed to stop trying to scare each other as Jenny declares Marnie the winner. Marnie gets up from the couch and goes to the kitchen to get something for them to snack on. As soon as Marnie closes the refrigerator door, she sees Ghostface standing behind Jenny and assumes it's another prank.
When Ghostface starts stabbing Jenny, Marnie soon realizes it's for real. When she sees that Jenny is really dead, she screams as the killer turns his attention towards her with his blade and slices her arm. A short chase ensues. She rebukes him as she runs, calling him a crazy bastard at the same time. She grabs the phone off the kitchen counter and races for the front door, trying to call for help. However, Ghostface runs around to the foyer where he knows he'll meet her and grabs her from behind. He stabs her in the stomach and she falls to the ground mortally wounded.
Marnie's last words are, "You're not real." before the knife dives down, making the titles come up.
Her death scene in the alternate opening is combination of Casey's and Tatum's. Once she realized Jenny wasn't pranking, the killer sliced her arm like Tatum's in the first movie. Similarities of hers and Casey Becker's chase scenes are both were held and stabbed from behind by Ghostface, both grabbed their phones and held onto them the whole time, and both were gutted then hung by their neck. The only difference is Marnie's chase was much shorter.
- "You're kidding. I don't get it." (referring to Stab 6 and Stab 7)
- "I get that but it's illogical! It begs the question that if the beginning of Stab 7 is Stab 6 then is the beginning of Stab 6 Stab 5 and if so, what is Stab 4 about?" (referring to the Stab 7 opening.)
- "There's a reason I don't watch these movies."
- "I don't see how you get off on this shi- (gargles)"
- "Gotcha!" (after pranking Jenny back in the alternate opening)
- "Please! I knew you were up to something!" (revealing she knew Jenny was pranking her in the alternate opening.)
- (Sarcastically) "Nice, assholes. Real original." (in the alternate opening.)
- "Okay, Jenny. You've already had your chance to scare me." (in the alternate opening when Ghostface is killing Jenny and Marnie assumes it a prank.)
- "What are you - are you crazy?! You bastard!" (after Ghostface kills Jenny in the alternate opening.)
- "You're not real." (to Ghostface before she is stabbed and killed in the alternate opening.)
- Her last name is the same as Scream 2 victim, Cici Cooper.
- Her casting call states: "Friend of Jenny's. Funny and mischievous, loves practical jokes. Sometimes pushes too far. Bored with Woodsboro’s obsession with the murders that happened when she and her friends were toddlers."
- There is a deleted scene where the police investigate the crime scene. It shows Marnie hung from the ceiling fan and Jenny bound to a chair, much like Casey Becker and Steven Orth from Scream. This is what Gale is referring to when she asks if there's any comment on these killings paralleling the original Woodsboro murders at the press conference being held at the hospital.
- Like the rest of the deleted scenes, the alternate opening was released with no score.