The Pretty Little Liars actress is guest-starring in her fiancé Patrick J. Adams' show Suits for the second time, but there's no love lost between their characters.
Bellisario, 29, plays Claire, a law student who met Adams' fake attorney Mike back when he was a bike messenger. When she returns, she's fighting against him as opposing counsel on his first case as partner at the firm.
"The thing with Claire is that the last time we saw her, she was a possible romantic interest for Mike, but she very quickly squashed that when she became the first woman that he lied to about going to school for law," Bellisario tells PEOPLE. "When she comes back, she's somebody who can pose a very big threat to him because she's not at Pearson Hardman kind of under his veil of trust and safety, where everyone's kind of agreed, whether they want to or not, to protect him. But she's kind of a free agent who has this information that could easily ruin him and make a lot of trouble for a lot of people."
Bellisario and Adams, 33, got engaged in winter 2014 after dating for three years. They met while co-starring in the play Equivocation in 2009, and they worked together again when he appeared in an early episode of PLL and she guested on Suits in a flashback in February.
"It's a blast," Bellisario says of their on-screen reunion. "It's just always so much fun to get to play around with your partner, because you have so much in life to draw on, and you have so much trust. When he tells me that something isn't working in a scene, I trust him to the end of the earth."
A frequent visitor to the Suits set in Toronto, Bellisario says she's happy to be put to work.
"I'm usually there kind of hanging around, getting Patrick snacks, so it's really fun for me to have something to do," she jokes. "I get to work with all these people that I really love, and I get to watch them do an amazing job every day and to get to contribute to that and work with these departments. It's so much fun, and it's a great opportunity."
See the couple's characters square off when Suits airs Wednesday at 9 p.m. ET on USA Network.