I think you probably have a point there, yeah. JB seems very easily influenced as an actor and he very much seems to adapt to whoever he's sharing a scene with.

I also think that's part of the attraction of Jack/Ianto for a lot of people, subconsciously anyway. JB and GDL work quite well together, because JB's occasional forays into pantomime disappear when they're met with GDL's infinitessimal eyebrow twitch school of acting. Whereas they're amplified by EM's rather OTT tendency to beat her chest.