This song's inspiration was relatively easy to find. This song was written at the time that Taylor was dating Joe Jonas. He claims he didn't cheat on anyone. But Joe dumped her after he met Camilla Belle/ There are lines which could indicate this, such as: "She's an actress/ She's better known for the things that she does on the mattress".
So the basic story of this song is that Taylor was either dating a guy or really liked him and this other girl got him alone and took that from her. As any girl would be, she's pissed off! She wants revenge. She makes all sorts of digs at this girl. You can see the lyrics here to see what I'm talking about. Personally I don't think the actress line is meant to be taken literally. I think she's trying to say that this girl isn't what she appears. She puts on a mask. She is whatever she has to be to get what she wants. But Taylor isn't about to let that get in her way. Part of this song is trying to get a message out to the girl that if she keeps acting this way, she isn't going to have many friends. More than just Taylor here will think badly of her. But this girl also has to know now that she has an enemy- and one bent on taking revenge on someone who stole from her.
But hey. If he's been telling you that he LOVES YOU... At least you're not the other girl.
-Autumn the Music Girl