First of all, Are you are wondering if this drama is worth to watch. The answer is YES! And I'm gonna explain you why since this is my first ever review (yeah that means it's worth my time lol)

1. I don't really like Thriller/Suspense dramas because I need to get attached to them from the very first episode which some of them failed to do. Big Mouth captivated me since the first minutes and kept me focused on the storyline until the last episode. It was so well written and the pacing was perfect (maybe the last two episodes felt a little rushed but that doesn't ruin nor affect the overall drama at all in my opinion)

2. The acting was 10 out of 10! Not only the main leads did an amazing job but the supporting characters as well. It was the comeback drama of Lee Jong Suk which delivered an outstanding performance (just like his previous roles) and we saw a more consistent and developed Yoona which to my opinion she did soo good!

3. The cinematography was on point and even though the drama doesn't have a wide OST, it didn't matter at all.

Final Thoughts!
Watch It. It is not going to dissappoint you. If you need some references from other dramas of the same genre, I would say is as good as Flower of Evil and Mouse.