Subjects, especially pronouns are usually dropped if they are obvious to everyone involved in the conversation.

Most of the time, it sounds rather unnatural to say the pronoun unless you want to emphasize it.

For example, I watch a movie in Japanese is:

But it sounds unnatural to native Japanese speakers depending on the context.

So to make it sound more natural, you omit the personal pronoun Watashi (I) and the particle Wa:

In spoken Japanese, it is also very common to drop the particle “Wo”, which indicates the direct object.

So we often say:

Pretty simple, isn't it?