This kdrama actually started off fine but mid-way into the story, I felt that the issue with Dr Seo's missing papers were lost amid issues within the "organization". I actually reached a point where I wasn't getting the whole point of the story.
Added to this the "big mouth" and "big mouse" monikers which left me confused as to what the title was.

Action wise, Big Mouth was more fast paced compared to Adamas, which was also airing that same week. But the push and pull as to who was Big Mouse kept leaving the viewer hanging.

I don't know why, but with Kdramas, endings are more often than not, very disappointing. Big Mouth's was no different. The last episodes all seemed like they were trying to fit everything within a 60minute time frame. When Miho got exposed to radiation, she didn't look sick and her passing actually felt very rushed. Changho also lacked emotion during this scene wherein he should have been devastated. Instead, he was surprisingly calm in his acceptance of Miho's fate.

These issues aside, I liked Lee Jong Suk here better than when I saw him in W. Visually, both him and Yoonah looked good together and hopefully, they will portray the roles of a couple in a romcom in the future.