Ianto's always been observant. When he was a child he used his powers of observation to study the people around him, to learn the nuances of social interaction. The social skills that came so instinctively to most were alien to him, and every expression and reaction he showed to the world was a practiced and rehearsed behavior. Ianto Jones, Torchwood employee, was just as fictional as any character in a paperback sci-fi novel. A construct of mirrored expressions and learned scripts that allowed him to conceal everything that was truly himself from the rest of humanity.
Only with Lisa had he tried to drop the act. Over the years she had gotten past the facade and begun to see the real Ianto, at least as real as he knew how to be. Lisa never seemed shocked when he said the wrong thing, never disgusted when he was unthinkingly callous, never judgemental when he was confused by the emotions of those around him. Lisa just accepted him.
Now he's been trying to let the mask slip in small ways, testing his co-workers against the standard forever set by Lisa. Jack had challenged him to be a part of the team so damn the consequences, he's going to be part of this team. Gwen's already failed that first test - she ran away from the naked pain on his face when he accused her of forgetting that they'd killed the woman he loved. Tosh is the surprise. She keeps passing his little tests, Tosh treats him now exactly as she did before she shot his love. Trying to test Owen has proven difficult given that Owen avoids interacting with him whenever possible. Ianto's just not sure if Owen's avoidance is a pass or failure.
Jack's become the real problem. Now that he's watching Jack he can spot the little tells, all those little signs he knows so intimately from his own behavior. Its surprising that it took Ianto this long to recognize that "Jack Harkness" is just as fictional as his own persona. He watches Jack watching him and knows that Jack is watching him watch Jack. It makes his head spin just a bit and the logic of who's watching who breaks down very quickly when he tries to dissect their game. Doesn't matter, all that matters is that they both see too much. Ianto wonders which one of them will break under the strain of lies within acts within pretense first.
Part of Ianto craves that confrontation. He craves another moment of being alone with Jack, of being just two monsters dropping their human masks and getting down to instinct. Ianto's mind plays back the feel of Jack's gun at his temple, the fear and anger churning in his guts and that clarity of seeing in that moment the real Jack. The replay makes his body tremble but not with fear - its anticipation, desire, and even trust. He trusts that if he pushes far enough Jack will pull the trigger and it makes Ianto feel safe like nothing has since Canary Wharf.
The monster hiding under Jack's cultivated smile might be a match for the one hiding in Ianto's tailored suit.