Watching a little marathon of Defiance today as I work and I keep getting pulled out of the story whenever Datak Tarr (Tony Curran) speaks Castithan. It sounds so stiff and clumsy in his mouth. The smoothness of a natural language is missing. His son Alak (Jesse Rath), though, speaks it with an efficient confidence that makes the language sound perfectly real, like any language of Earth with which I am simply not familiar. I notice the same when watching GoT. Some of the actors speak Valerian beautifully, others suddenly develop a speech impediment. I feel like, if the actor who is meant to be a native speaker can't pull the fictional language off so that it sounds smooth, natural and real, the actor shouldn't be given lines in the fictional language or placed in a role where it's needed.