When she started out in the game she was not as grand as she is today. She has honed her craft well and truly. Which just goes to show, practice and persistence pays off.
Julia Roberts began her acting career in small roles she landed mainly because she is beautiful. She is very tall and very thin and she is pretty in an un-threatening way. She is nice to look at but she doesn't make other women hate her out of jealousy. On the contrary, she looks like someone we'd like to be friends with.
After a few roles in films like Satisfaction and Mysitic Pizza she landed the role of Vivian in Pretty Woman. Arguably the role she is most well known for. She carried the whole film with her sassy star power, but, she showed her lack of experience a tad when she was forced to improvise. A truly good actor is in the zone so much that when their co-star sways from the script they can hold tight and improvise in character. In Pretty Woman, during one of the most famous scenes know as The Necklace Scene, Julia Roberts drops character. When her co-star Richard Gere improvises and snaps the necklace case shut on her hand, she laughs, but, (I am sure) she also looks at someone off screen, probably one of the crew. It's just for a split second but to the semi-trained eye, it is noticeable. You be the judge.
One of my favourite performances is her portrayal of Erin Brockovich in the film of the same name, the role that won her an academy award in 2001. Erin Brockovich is one of my favourite films to watch over and over again. I love a movie where the underdog wins.
But for the sake of this argument I must say that the real proof that Julia Roberts can indeed act is below in the clip from her most recent film The Normal Heart. In this film she plays a wheelchair bound AIDS researcher, back before it was even called AIDS. It's a great film. All of the performances are amazing but Julia's is unflappable. I wouldn't be at all surprised if she won an award for this.