Gossip Girl was a smash hit of a TV show, and it definitely helped to kickstart the career of Blake Lively, but she never thought too kindly of her character Serena van der Woodsen. In what we suspect to be a common problem with actors, the nature of the part clashed with Lively's strongly-held beliefs and convictions, which sometimes led to difficulties with her delivering performances.
As a socially conscious performer, Lively has said that she wanted to bring positive messages to people, and van der Woodsen wasn't an ideal vehicle for her to do that. Although she's glad that people appreciated the character, and her performance, she personally felt compromised by some of the things she had to say and do. That may go some way to explaining why she appears to be much more selective about the parts she's agreed to take on since.