Blake Lively has got one of the best "Social Smiles" in Hollywood. That is, her "acted smiles" in social/interview/public appearance settings - most of the time comes pretty close to resembling a "Sincere, Felt Smile". Very few people - even actors/actresses can truly pull this off. That's one reason why this particular image is a great example here. Her mouth shows a trace amount of fear - not the typical type of fear though. She's trying to smile, but she can't in this particular moment. Her bottom teeth are showing and they shouldn't be with a sincere smile, but it is a characteristic of fear. In this case, it's an empathy-embarrassment-apology type of fear. The kind of fear where she has just delivered some bad news or she has to back out of a commitment, etc. It's not danger-fear, but it is a type of fear. One signal that it's not a classic fear is that her eyes are not opened wider than normal (e.g., with "whites of the eyes" showing above the irises/irides - not just on the sides of the eyes) in fact her lids are a bit more closed.
Blake's central eyebrows are also elevated. Another sign that goes along with this - is her central forehead being contracted. This is a classic signal of pain, either physical or emotional pain and either directly felt or experienced through empathy.
This whole smorgasbord of signals is what is known as a gesture cluster, when several body language signs are seen simultaneously or nearly so - like words in a sentence - they take on a specific meaning. So, in summary, when you see this mild "yikes" type of expression displayed by the mouth, the eyes closed slightly, the central eyebrows raised and the central forehead wrinkled - you know that she/he is about to deliver some news that they know you don't want to hear (and they fear speaking it). You may very well see this beacon several minutes before the news is spoken - which will give you time to mentally and emotionally prepare. Your own body language, tone and words will then work to your advantage. Our overly emotional response to bad news is a common flaw that many share and a few seconds warning is priceless.
Secret # 133: Surprise or Smile or Neither?
Analysis # 2: Body Language of Blake Lively and Penn Badgley Strongly Suggests an Impending Break-up in Near Future
Nonverbal Communication Secret # 2091: Sincere Smiling (a la' Blake Lively) and Laughing Reduces Anxiety and Heart Rate, Boosts Immune System and Relieves Pain
Nonverbal Communication Analysis # 2203: A Body Language Metric of Intimacy and Affection - How Do YOU cuddle?