I was thinking that a person has a different behaviour in real life compared to a movie character. The dialogue has a different content and many little things such as being comfortable around your friends or family. In movies, the actor is a bit stiff and everything looks a bit too scripted. Should actors be a bit stiff or should they be more natural, more like in real life? 

Yes but in real life you are living in the moment. In the movie you need to send an emotion to the audience. It's different context

In real life, you are the actor and you are responsible for your actions and there are consequences, too. In movies, there are no consequences, because everything is scripted. It is different when you live in the moment and when you pretend to live in the moment. There is also an audience, watching and judging every minute. The audience can also rewatch the movie if they wish. In real life, there is no pause, stop or replay button. Everthing happens only once. 

In real life, you don't have to film the same scene many times to get it right. I think it is hard for the actors because they are required to redo the same scene over and over again. As for the way they act, there is a difference between real life and movies and honestly I would too love to see more acting like in real life.