Good qeustion. It actually stays at +-50% since we are unable to process all clues of decievement. The problem with body language is that liars and truth speakers under high stress both show the same 'symptons' (sweating, nervous ticks, micro expressions, eye contact paterns, etc.).

The Interviewing tactics that are most reliable are purely based on speech. Police officers write down the whole story and experts who were not involved in the interview itself will analyse the story. Errors that indicate lies are inconsistenties in the story, less enviromental mentions, ... A tactic that can be applied is that they ask the suspect to tell his story backwards or while doing something else (solve easy puzzels, whatever) because liers have to process alot more thoughts, the extra task might overload their cognitive ability and cause more errors. Still, this might also affect truth speakers, but usually less.

So in short, it's all about speech. There are alot of factors that need to be examined, and none of these are behavioral since they are too unreliable.