Actors pretend to be someone else in movies. That's their job. Some are more successful at this, while others are less successful. Sometimes they play their role so convincingly that we forget we are watching a movie. Let's take some biographies for example. Did you notice that you are actually looking at Helen Mirren and not Elizabeth II. in The Queen or Daniel Day Lewis and not Abraham Lincoln in Lincoln? These actors didn't just play the part, these actors became the person they portrayed. See the amazing similarities between the actors and the real people who played them.
