Adventureland

In 1987, James Brennan (Jesse Eisenberg) has two plans. The first plan is to have a summer vacation in Europe after graduating with a comparative literature degree from Oberlin College. His second plan is to attend a journalism graduate school at Columbia University when his holidays end. A few days after his graduation, his parents (Wendie Malick and Jack Gilpin) advise him to seek a part-time job rather than going to Europe when they unexpectedly announce that financial problems have taken a toll on them and they would be unable to financially support him.
James gets a job at Adventureland, a local amusement park in his hometown of Pittsburgh, Pennsylvania, where his childhood friend Tommy Frigo (Matt Bush) works. Assistant manager Bobby (Bill Hader) assigns James to the games area. He meets his co-workers: sarcastic Joel (Martin Starr); Bobby's wife and park manager Paulette (Kristen Wiig); Sue O'Malley (Paige Howard); Mark (Mark Miller); the alluring Lisa P. (Margarita Levieva); and the park's technician, Mike Connell (Ryan Reynolds), a part-time musician. Another games worker, Emily "Em" Lewin (Kristen Stewart), saves James from being stabbed by a lying, cheating customer.
With her father and stepmother away, Em throws a party and gets to know James. During the conversation, Em persuades James to join her in the house's swimming pool. After Em leaves the pool, Frigo humiliates James by announcing to the party that James has an erection while leaving the pool, causing an embarrassed James to jump back into the water. After the party, Connell, who has been having an affair with Em, comes over to further pursue it.
Later that week, James goes for a drink with Em and she is surprised to learn James has never had sex. They share a first kiss. The following day, James tells Connell about his strong feelings for Em, which Connell reports to her. Em tells James she wants to take things slow due to problems in her life, leaving James confused and upset. Sue drunkenly makes out with Joel, but rejects him the next day, saying her Catholic parents would not allow her to date a Jew. Outraged, Em calls Sue an anti-Semite in front of other staff members in defense of Joel. Lisa P. asks James on a date, but he has mixed feelings because of his relationship with Em. After Connell talks him into going, he accepts Lisa P.'s offer.
After the date, during which Lisa and James kiss, James learns Em had called to say she regrets having rejected his feelings. Joel later sees James and Em walking together and, irritated by the chain of events, quits. James unsuccessfully tries to talk him out of it, and Joel reveals he's angry at James for dating Lisa P. when James is already in love with Em. James tells Em about going out with Lisa P. After hearing of this, Em goes to Connell's mother's home to end their affair. The park's mentally challenged parking attendant, prompted by Tommy, tells James he saw Em and Connell doing "pushups without any pants on" in the back of Connell's car.
James goes to Connell's mother's house, which is where Lisa P. told him Connell takes girls to have sex, and sees Em leaving. Shocked to see James, Em becomes tongue-tied. James leaves angrily, and Em cries. James tells Lisa P. about the affair and asks her not to tell anyone, but she tells her friend Kelly. Upon noticing that all the park employees know, Em quits and moves back to New York. A heartbroken James drunkenly crashes his father's car into a tree and passes out. The next morning, his mother angrily wakes him up and tells him he has to pay to repair it with his summer earnings.
Now, without enough money for graduate school, James nonetheless heads to New York City with his parents' blessing, and waits outside Em's apartment. She is reluctant to talk to him, feeling she has ruined everything. James tells her he sees her in a different way than she sees herself. Touched, Em brings James up to her apartment. James reveals he is not going to Columbia this year, and considers next year. He gets out of his wet clothes and finds Em still has an Adventureland shirt. They kiss and begin to take their clothes off. James asks, "Are we doing this?", and Em says, "I think so."

Question: Who has been having an affair with Em?
Answer: Connell