First thing, you need to group your alternative fields together. It will be a hassle for a user to move around back to the first field if he doesn't want to fill the third field. You should not move the user up and down the form for filling the details. It should be a top down approach. So I would suggest you to group your link and comments fields together.
Put the "What movie is it about?" first, followed by the two detail fields.
I would also suggest you place a small note before the two detail fields, like "At least one of the following two fields are required".
I also thought of using an 'OR' between two fields but it would look like that you need to fill only one out of the two and the user wont be compelled to fill both the fields.