Julia Roberts is really good in this!
I've just re-watched "Notting Hill" for probably the fifth time and I think this film really does show what a fine actress Julia Roberts can be. She has amazing inflections in her facial expressions that at times can be very subtle, showing vulnerability or other emotions.
She also starts out guarded and wary, as you might expect someone famous to be, and then gradually opens up as she sees that the "commoner" Hugh Grant might be a good romantic partner for her. This culminates in the last scenes, albeit sappy, but also wonderful and perfectly synchronized with the lyrics of the Elvis Costello version of "She." When she realizes that "William" wants to be with her she just beams, and does so through the wedding and film premiere clips as well.
It's like she finally found the real happiness in a "real life" she was missing when she was just a famous person. I actually think this makes it more believable (to the extent that the film is believable at all!).