New Girl‘s new leading lady Megan Fox is fitting right in – so well, in fact, that she’s already hopped into the shower with one of her cast mates! According to costar Hannah Simone: “[Megan] and I got to do a lot of work together against the boys, which is always fun and then by the end of the episode, they threw us in the shower together.”

Simone, 35, revealed to PEOPLE at New Girl‘s 100th Episode Cake-Cutting Celebration in Los Angeles last Wednesday that “Megan’s character and my character have a romantic history” – and it’s apparently not so squeaky clean. Naturally this news sends Schmidt (Max Greenfield), the fiancé of Simone’s character Cece, “into a mᴀssive spiral,” teases the actress. “He doesn’t think that’s H๏τ or exciting. It’s just like, ‘Oh my God. Panic attack.’ So that was really fun.”

As for Fox – who has been filling in for a portion of the fifth season during Zooey Deschanel’s maternity leave – jumping right in has been a blast. “I’ve had fun from day one,” Fox, 29, told PEOPLE. “I wasn’t sure how it was going to be because when you walk on to somebody else’s set, a tone is already in place, and you never know how you’re going to fit into that.”

Happily, the love has been mutual. Costar Jake Johnson said the actress has “been so funny on the show and so good.”

“It has been a very pleasant surprise,” continued Johnson, 37. “She’s also ego-less in a way, that doesn’t mean she’s not confident, but she doesn’t bring any extra stuff to a set. Without having known her personally at all that was a very welcomed thing to see, and our cast and crew have all really appreciated it.”

New Girl returns to Fox for season 5 on Jan. 5