Sally Stageplay's battle phases take the form of four scenes that make up a complete production, and the player must defeat her in all four phases to win.
Sally is a human actress who has blonde hair with three puffs that are tied with three hairbands and tan skin with long arms and legs. She wears cartoonish white gloves, a pearl necklace, pink earrings and a turquoise dress and boots. In her third phase, she appears as a cardboard cutout with red hair and black wings that now wears a red dress and a golden helmet. In her final phase, she wears a white dress along with a golden halo and blue angel wings.
Sally loves attention, calling herself a starlet and telling Cuphead and Mugman to get off the stage if they lose to her. When it comes to acting, she seems to take her job very seriously to the point of treating her battle with the brothers as a part of the play she's in.
In the good ending, Sally can be seen kissing the man who played as her newly married husband at the performance, showing that she does have feelings for him.
Sally is seen wearing a bride's dress before ripping it off to reveal she's wearing a turquoise dress and boots and armed with a orange-red parasol as the curtain rises up to reveal a wedding scene.
"Break a leg...nah, break two!"
—Sally Stageplay, Phase 1 Death Screen
In this first phase, Sally has four attacks that she alternates back and forth or uses one attack more often than other:
- Her first known attack involves her parasol swallowing her which she later falls from above and lands on the players damaging them. Where she lands may or may not be based on where the players are.
- She occasionally stops to blow the kiss as the kiss travels at the direction she is facing. The kiss can be avoided as it will fly off screen or parried to neutralize.
- She will occasionally jump up in the air and strike the players by diagonally kicking at them, or she will remain in the air spinning and throw fans at the player. If the fans miss the players, they will be set on the ground as a trap for players accidentally running into them.
After taking enough damage, she jumps into the background and drives away in a car which then starts the second phase.
"Stay away from center stage or succumb to the power of a starlet's rage!"
—Sally Stageplay, Phase 2 Death Screen
The second scene sets in front of what seems to be Sally's house. In this phase, she still use her diagonal kicking attack and fan throwing attack, but she now uses a new attack replacing her other previous attacks.
Her new attack involves spinning her parasol to release 4 wind-up mouse toys that will travel along the ground, on the wall and the ceiling. Once they reach the top, they will drop down on the players when they are underneath them.
While on the background, a child and a little girl (presumably to be Sally's children) will damage the players by throwing a milk bottle down on the players through 8 windows including the arch above the door. After taking enough hits, her dress now changes into an angel and she is being carried away, making it looks like she dies after fighting.
"Good riddance -- go away! It is time for my soliloquy."
—Sally Stageplay, Phase 3 Death Screen
In this next scene sets in the sky, players now face her goddess form, which made out of cardboard and controlled by the people above.
She now has three brand new attacks that differ to her previous. Before the battle begins, there will be a cardboard cutout at the upper left corner showing which kind of attacks is being executed:
- A cardboard meteor carried down from above which players can destroy to reveal a pink star. The pink star can be later used to jump over the big wave.
- A cardboard tidal wave being carried by a person that follows after the meteor attack. To avoid this big wave, players need to parry the pink star, left over from the meteor attack, that they can jump over it. It may also be dashed through if the player is using the Smoke Bomb Charm.
- Cardboard lightning bolts that carried down on where the players standing before pulling back up. A maximum of 4 lightning bolts can be present at the same time. These will not appear on simple difficulty.
Once players deal enough hits, the goddess form is defeated and the final phase can begin.
"Please exit stage left during my standing ovation!"
—Sally Stageplay, Final Phase Death Screen
In this final, yet slightly messed-up scene, Sally returns in her angel outfit to receive a standing ovation from the crowd as she is being carried from left to right and back.
She does not have any attacks, but rather her parasol slowly falls down from above spinning on its tip and chases the players relentlessly all while they have to shoot Sally above them.
Not only the parasol chases the players down to damage them, but the crowd will also damage them by throwing roses onto the stage. These roses will fall on a random spot where players might accidentally run into them, though some of them will be colored pink and can be destroyed by parrying.
After taking few more hits, Sally is down for good as she would have a ripped off angel dress and a black eye when she was defeated.
In simple difficulty, Sally will be defeated in her goddess form before the ovation can happen, so this phase is unplayable in that difficulty.
- The crowd is shown clapping whenever a phase is completed, one person in the crowd is even shaking a cane around.
- Sally Stageplay seems to be inspired by Olive Oyl from Popeye in terms of her body much like Hilda Berg.
- Sally, along with Captain Brineybeard are the only two human bosses in this game.
- Dr. Kahl's Robot does not count, as the robot and Dr. Kahl are different entities, even though in one phase Dr. Kahl controls the robot.
- In Phase 1, there is a man playing the role of Sally's husband (who happens to have a real relationship with Sally as seen in the good ending) dancing around in the background. When Sally takes damage, he pulls his hat to his head. When Cuphead takes damage, he jumps up in delight. He is also seen in the second phase hiding behind the bush. When second phase is completed, he starts crying.
- Sally Stageplay is the only "normal" boss to not have transformations. Her third phase is only a cardboard version of her (which is done in cutout animation) and the attacks are made by the other actors. The big waves are carried by the same man from two other phases.
- Of all bosses in the game, her third phase is the only boss battle that has no thick cartoon outlines.
- This is the first time Studio MDHR includes its name on something in the game besides the death screens.
- Her death line is a reference to the idiom "Break a leg" which means to do well. She takes it literally after saying "nah", and saying to break both legs.
- Every time the curtain falls, there is "ASBESTOS SAFETY CURTAIN" written on it. This references to the mineral Asbestos formerly used to make safety curtain before it's widely known to cause lung cancer, mesothelioma and asbestosis.
- If the player parries one of Sally's kisses during the first phase, it is possible to stand on top of the cherubs hanging from the ceiling.
- The third phase of her fight is set much like the boss fights in turn-based RPGs like Final Fantasy, with her cardboard cutout staying immobile and panels with the names of the attacks showing up above her.
- Her cardboard form has a similar appearance to that of Kefka in Final Fantasy VI.
- Her boss intro is similar to Urien’s from Street Fighter III.