The doppelganger from a parallel world has been posing as the real Olivia throughout the third season of the Fox sci-fi drama.
"Peter pretty much believes that's his Olivia," Pinkner told E! Online. "He's just clocking the changes that are going on with her... since she's been through so much and has come back. He [sees] normal changes that are more about her personality, rather than suspecting she's not who she says she is."
Co-producer Joel Wyman also confirmed that Peter and the impostor "definitely [had] sex" at the conclusion of recent episode 'Do Shapeshifters Dream of Electric Sheep?'.
"That was sex... and the next time we see them together, which will be in two episodes from now, pretty early on in the episode, we define exactly where their relationship is right now."
Fringe returns to Fox after a two-week break on November 4.