first off, by hot you mean what temp? CPU and/or battery temp.
now, while you are watching movies, its normal for it to warm up quite a bit. if you add to that being plugged in charging, it can get down right hot. but there are 2 safeties features in place so that you dont worry. first is the thermal throttle, which will reduce your CPU speed to cool down your device. if that doesn't stop the warming up, you have the battery temp safety shutdown and the CPU temp safety shutdown. battery temp shutdown is when your battery reaches 60C, and CPU temp shutdown is when your CPU reaches 105C. but really, just realize that when you are charging, and using your phone, it can get really hot, and that's normal.