Why does my blu ray movie switch between widescreen and full screen between scenes?

I just bought The Dark Knight, my favorite movie, on blu ray. I put it
in my ps3 and started watching it on my vizio HDTV(1080p). While
watching I noticed that,even though it all was in hd, some of the
scenes were full screen and some of the scenes were widescreen. Why is
this? Is it my tv? Or the way they filmed the movie? Or even both? I'm
just curious why it was like that.