From my experience body language will either usually break it or make it. Though sometimes when you start off it is hard to achieve the right body language without having the inner self confidence. I found there are some cheats to that. Just like it is always advised not to go on a date before talking to some people about random stuff just to warm up. I think it should be advised for beginners to do some body language warm-ups as well.
There is a great TED talk on this at "ted. com/talks/amy_cuddy_your_body_language_shapes_who_you_are?language=en". Pretty much says to do some power stances before an interview. Your stances help affect your mindset. I've found it works quite well with improving the body language of guys just starting off PU as well.
P.S. Can't post the link to the TED talk as a new user