Obsessed

Derek Charles (Idris Elba) works for a finance company and is married to Sharon (BeyoncÃ© Knowles). While Derek is at work, he greets temporary worker Lisa Sheridan (Ali Larter), who, believing Derek was flirting with her, attempts to seduce him throughout the film. Derek repeatedly rejects her, but Lisa continues to pursue him, making sexual advances on him at the Christmas party and flashing him in his car. Derek intends to report Lisa to his firm's human resource management, but learns that she has quit her job. Thinking that Lisa has given up, Derek is annoyed when he receives flirtatious emails from her. Derek and his workmates visit a resort for a conference, where he spots and confronts Lisa, who spikes his drink. Incapacitated, Derek is helpless when Lisa follows him into his hotel room and rapes him. He confronts Lisa again the following day, and hours later discovers her lying naked in his bed after attempting suicide by drug overdose, and calls for medical help.
After repeated attempts to reach Derek on his phone, Sharon finds Derek at the hospital and suspects that he and Lisa had an affair, as Lisa claims. LAPD Detective Monica Reese (Christine Lahti) at first questions Derek's fidelity to Sharon as well, but soon becomes skeptical of Lisa's claims, due to inconsistencies in her side of the story, and informs Derek of her belief in him. Sharon, refusing to believe Derek, kicks him out of their house, and he moves into a separate apartment. Months later, Derek and Sharon meet up for dinner and finally reconcile. Meanwhile, Lisa tricks the babysitter Samantha (Scout Taylor-Compton) into letting her in under the pretense of being one of Sharon's friends. When Derek and Sharon return home after dinner, they discover that Lisa had been in the house and seemingly abducted Derek and Sharon's infant son, Kyle (Nathan and Nicolas Myers). Derek goes to his car with the intent to pursue Lisa, only to find the baby is safely sitting in the back seat. He and Sharon immediately take Kyle to the hospital for a check-up. When Derek and Sharon return home from the hospital, they find Lisa has trashed their bedroom and removed Sharon's face from their family portrait. Sharon leaves a threatening voice message on Lisa's phone, and she and Derek set up a home alarm system.
Lisa learns that Derek and Sharon will be going away from town for a few days, with Sharon leaving one afternoon and Derek the next day. While Sharon is on her way to pick up Kyle, she realizes she forgot to set the alarm system and returns home. Meanwhile, Lisa breaks into Derek and Sharon's house again and decorates the master bed with rose petals. While setting the alarm, Sharon hears Lisa in the bedroom. She confronts Lisa, who tries to convince her that Derek was seeing Lisa. However, Sharon sees through her lie and attempts to call the police, but Lisa tackles her to the floor, and the two engage in a fistfight. Derek calls the home phone and Lisa answers; he calls Detective Reese and they both head toward the house. Lisa runs to the attic and Sharon pursues her. Sharon leads her to a weak spot in the attic floor, where Lisa falls through. Seeing that Lisa is in mortal danger, Sharon reaches out in an attempt to grab Lisa and lift her up, but Lisa refuses and tries to pull Sharon down with her. Seeing that the floor is beginning to buckle, Sharon pries Lisa off of her arm. Lisa falls and smashes onto the glass table below. A chandelier falls from the ceiling onto Lisa, eventually killing her. Detective Reese arrive as Sharon comes out of the house. As soon as Detective Reese goes inside the house to investigate Lisa's actions, Derek soon arrives; he and Sharon then tearfully embrace one another.

Question: What does Lisa believe Derek is doing?
Answer: Flirting with her