By increasing heat, we don't actually mean that director Sidney Lumet made the actors in 12 Angry Men work in a sauna (although that might have been fun). But what we mean here is that Lumet made the conscious choice to start us off with a lot of wide-angle shots of the jury room. These shots allow us to see everything that's going on at once, and they remind us that this movie was originally written as a play.
But as the movie unfolds and the tension builds, Lumet brings his camera closer and closer to the actors' faces, making us feel claustrophobic in the process and building tension without really letting us know what he's doing. By the end of the movie, all we can really see are the sweat-covered faces of the jurors yelling in each other's faces. And you know what? This totally brings out the uncomfortable reactions that Lumet wants.