Usually with night terrors, you can't really connect with them - their eyes may be open but they don't really see you, don't hear you.
Nightmares are different - they wake up and, while they may still be crying or upset, they know you are there.
But 15 months is young for both of those things. Not impossible, of course. I think my niece was closer to 2 when she had them.
I've never, ever heard of a 15 month old who can watch a movie from start to finish, even if it's only 1/2 hour (and most movies are longer than that). He's watching it because you leave it on but he has no sense of the story being completed. You could easily turn it off sooner. However, I don't want to sound like I think this is a good idea. Teaching a child to watch a screen until he falls asleep is creating a horrible habit you will absolutely regret for the next 17 years! You need to calm this child down through quiet reassurance and calm sounds, not stimulate his brain with action and words and bright lights! Of course he’s tired the next day – his brain isn’t getting vital sleep! Please, please talk to the pediatrician about how vital a good night’s sleep is for a child’s brain development! And if you need to, video your child in the middle of one of these episodes of screaming so you can show the doctor.
Are you sure you aren’t putting the movie on because you are bored or frustrated with a non-sleeping child? I know how aggravating this is and you just want to do something, anything. But you could be doing so many other more helpful things. You need sleep too – it’s dangerous to go through the day totally sleep-deprived. If a child has a night terror, usually you just hold the child or gently stroke him until he comes out of it. If you have to protect him because he’s active, fine. If he’s just waking up because he’s teething or has reflux, then you work to eliminate the pain (but you don’t just give Tylenol every night either – you don’t say how long this has been going on). I know a crying child can wake the whole household, but you have to take turns soothing him and helping him learn how to go back to sleep, alone and without being cuddled into a deep sleep.