Yes, you can use 'Definitely' as a response. I'm not sure that you'd say it as a response to a subjective opinion about a movie, though. You might say: 'I'm hoping that enough people come to the meeting tonight. You're coming, aren't you?' 'Definitely'. Here you'd say it to reassure your friend that you really are going to come.

In American English, "Definitely" is often used as a response in informal conversations with friends. It is used just the way you used it. "For sure" is another informal response. They mean the same thing.