I'm watching Batman with Sara.
or
I'm watching Batman, with Sara.
Sara = my friend, not an actress. Which is correct?
I'm watching Batman with Sara. - This would indicate you are physically watching Batman together with Sara. If Sara is your friend, I would leave the comma out.
or
I'm watching Batman, with Sara. - This would imply that Sara is an actress in Batman.
Example: I'm watching Grease, with John Travolta.
In the sentence without the comma, the sentence construction closely associates Sara with either you or Batman. For example, you and Sara are watching Batman together. Less idiomatically, you could be saying that you are acting as a chaperone for Batman and Sarah.
The comma interrupts the second sentence, so the second version asserts that you, as an individual, are watching Batman. It holds Sara's involvement as a separate idea. Depending on context, it can emphasise her relationship with you or Batman, or to assert her lack of relationship with you or Batman.
Note that writing Batman in italics doesn't necessarily make it the title of a movie.