Every detail of The Invisible Man's ending has significance, including why Cecilia chooses to eat steak at dinner. Leigh Whannell's movie adaptation of the H.G. Wells novel of the same name reimagines the Universal horror character as a handsome but controlling optics engineer Adrian Griffin. As a fun Easter egg, Griffin is the first name of Wells' title character. Instead of terrorizing a small village, Adrian focuses on his ex-girlfriend, Cecilia Kass (Elisabeth Moss). Unlike Wells' Griffin, whose inability to reappear drives him mad, Adrian strategically chooses to wear the suit, making his invisibility a matter of choice. It gives him a way to make good on his threat to Cecilia that if she ever left him, he'd find her, and she wouldn't know until he gave her a sign.
Adrian's sign is the prescription bottle of pills Cecilia uses to drug Adrian so she can escape. Adrian's gesture only confirms what Cecilia has feared all along: Adrian isn't dead. Adrian wearing his suit to gaslight Cecilia quickly escalates into acts of violence. His most grisly and shocking crime is slitting Cecilia's sister's throat in a crowded restaurant and framing Cecilia for the crime. Adrian's brutal act has the intended effect. Cecilia's efforts to prove her innocence by claiming her ex is invisible and stalking her makes Cecilia sound insane. The death of her sister is a pivotal event that motivates Cecilia to stop being the victim and begin thinking strategically to expose Adrian for the monster he is.
The showdown between Cecilia and Adrian takes place over dinner. Cecilia agrees to meet with Adrian to discuss the possibility of a reconciliation. Adrian offers Cecilia her choice of sushi, steak, or pasta, and Cecilia chooses steak, a seemingly innocuous and inconsequential detail. Cecilia picks the option that most likely appeals to Adrian, which also happens to be the one that guarantees he'll have a knife nearby. Cecilia's real motivation is further obscured because one of the options is sushi: pregnant women can't eat uncooked fish. This could be an oversight on Whannell's part, but given how carefully he constructed the movie's plot, leaving little to chance, that feels unlikely. Ultimately, Cecilia uses Adrian's knife to slit his throat. Cecilia craftily sets Adrian up to make his death look like a suicide, and he unknowingly makes it easier for her by offering steak.
It's an easily overlooked detail since it seems like Cecilia's only purpose is to try and get Adrian to confess to his crimes. She even has James listening nearby. Adrian maintains his innocence, but there's a sinister quality detectable beneath the sincerity. Adrian never openly admits to anything he did, but Cecilia predicts Adrian is narcissistic enough to want to take credit for his elaborate scheme. Adrian claims to know Cecilia better than anyone else, but he's so singularly focused on seeing their relationship from his perverse perspective, he fails to realize he doesn't know her at all.
Whannell's final scene plays out like a chess match with Adrian and Cecilia; each of them are strategically planning their next move. Adrian's carefully worded confession allows him to give Cecilia what she wants without implicating himself. He believes that, once he does this, he'll get her back. He misreads her reaction, enabling her to slip away and, ultimately, the suit Adrian wears to terrorize her is used against him.
Cecilia wants Adrian to confess because by doing so, he reinforces the fact that she's not crazy. Cecilia never doubts Adrian's guilt, but the entire ending revolves around Cecilia regaining control over her life while simultaneously exacting revenge against the person who took it away. Throughout The Invisible Man, Adrian stalks Cecilia like prey, but in the end, she becomes the hunter.