Just like any system that needs cooling it requires a source for cool fresh air.
In the case of the Mac laptops it's through the keyboard. So when you block the air from entering through the key board the fans need to work harder. I would recommend leaving the lid open about 1/4 inch or more to allow the air to flow.
The other factor here is how hard the system is running. Watching a TV show or movie is a heavy compute process so it creates a lot of heat. Using Flash or accessing heavy graphic elements on a web site can likewise push your system.
If your system is clogged up with dust this will hold the heat in and cause the fans to work harder. Cleaning out your logic board and fan of any dust buildup will help. Lastly, redoing the thermal paste between the CPU & GPU units and the heat sinks will allow better cooling if the current paste is dried out or not correctly put on. There is a bit of skill here in doing it correctly. you don't want to much or to little, and you don't want any air bubbles either.
I would recommend getting 'Temperature Gauge' and 'Monitor Deluxe' apps from the App Store to monitor your system with the lid open Vs closed so you can see the difference in its' temp. Use Monitor to see whats happening with your CPU load in case you have something running wild.