Yes, it might test your patience in the beginning. It seems like a slow paced movie,where you don't even feel like doing your guess work. I also have read some reviews on acting of lead roles,but I found them good enough. It might not be a seat edge movie, but gives you that satisfaction of watching a good suspense thriller. Spanish movies has good number of thrillers, sure you won't get disappointed, Dispite being a slow paced movie, if you wait until pre climax it will surprise you enough. It gives you that required plot twist and you'll love the movie once you finish watching it. So I suggest you to go for it!
