Is it possible for a host to go beyond it's storyline in westworld?
A host can gain free will / self-detemination
A lot of this is covered in this answer over on SF&F.
However, since then it has been confirmed by the showrunners.
Jonathan Nolan @ Vulture.com
Nolan explained, Steadicam shots indicate that hosts are doing what they've been told to do, but handheld shots indicate that they're acting on their own.
"In the finale, when Maeve gets onto that train ... the Steadicam is leading her over. Now, it’s just keeping pace with her as she makes the decision. What we understand in the moment is it’s the first real decision she's made all season. Which is, she's not going to fulfill the script she's been given, which is to take this train wherever it's going, and do whatever else she's programmed to do. She can get off the train. At which point, we shift to handheld camera, which we'd held back on throughout the entire season until one moment with her, and one moment with Dolores, when Teddy comes to rescue her. We get Maeve off the train with a handheld camera. And I remember watching the dailies and almost being shocked at how effective a cinematic technique can be if you hold off on it for long enough. If you dial it in at just the right moment, that suggests she's literally like a train coming off the tracks. We're no longer in programmatic or prescribed behaviors. She's improvising, and we're right there with her."