I am trying to get into Fringe and I don't think Josh has a lot of chemistry with the actress who plays Olivia. I heard from various places that they did not get along very well offscreen and it made me believe this is why they struggle with their chemistry. I think Josh is pretty open, honest and expressive. Because he is so honest about who he loves to work with or thinks highly of, he puts that energy or feelings into his work. That's why he was so excellent with Katie. He really held her in high regard and loved working with her. He's also excellent on Fringe with Walter, because he really loved that actor offscreen so their father/son relationship shows up amazing onscreen.
I think Katie got along well with James, but not at the same level as she did Josh. It's obvious because their platonic scenes were good but their romantic scenes were bad. Towards the end of the series, Katie started to do a lot of closed mouth kissing with Dawson and Eddie, it was so weird to watch. She was pecking her way through makeouts with Dawson. Their entire first time time together, the makeouts leading to the sex were all closed mouthed. I am so glad that didn't happen with Pacey.