My sister has the exact same problem with TWO of her laptops; it's usually because your laptop is small and portable, meaning a weaker fan system. Because of this, it's unwise to leave it on for long periods of time (even if it's on hybernation or sleep mode).
What we usually do instead of paying for a new fan is feel the empty spaces beside your T-Pad (because this is where you place your wrists/arms to type if you place it on your laptop, meaning body-heat will be transmitted here), on your keyboard (usually the side opposite to the fan sound), and on te small space between the keybaord and your laptop screen's hinge. If any of these areas feel extremely warm, warmer by your body temperature by a certain margin, or close to smoking hot, then go and grab an icepack and place it there. Move the pack around after one area has gotten cooler, this should pro-long the 'life' until you finish your movie/project and let your laptop rest.
Personally, I've learned that keeping your laptop on a glass table would make the situation even worse and if you have a nice AC system, keeping that on helps bunches.
Small and convienient laptops are usually the ones with fan and inner-system problems the most, of course depending on who's using it and if they let it rest or not. Big laptops aren't very nice to carry into bed, but they work oodles better if you are forced to stay on for a long time due to work, writing, or hobby-wise. It's one of the reasons why gaming laptop brands such as Alienware last for so long
(in my experience).