I think the main problem of this movie is, that trailers kind of ruin the intrigue. So do not watch those. In fact, do not even read the synopsis, if you can. Neither will tell you the details, but they will tell you enough for you to start expecting things. I think it is much better if you just know that this is a movie about a friendship between 2 families, that met each other on a vacation, and how that friendship goes sour. If you watch it knowing only that it may be a bit more entertaining.
Even if you do not follow my advise, it will still eb a good watch. The most prominent part of the movie is the atmosphere that they achieve by some little details, lighting and camera work. And through McAvoy. He is definitely the main attraction here, and for a good reason. Does that justify the length of the movie? Probably not, since it could have been like 20 minutes shorter, but still ends up a decent... "Thriller". Let's leave it at that.
