The obsession is with building a better bridge, and finishing it on time. The story's great irony is that once Nicholson successfully stands up to Saito, he immediately devotes himself to Saito's project as if it is his own. He suggests a better site for the bridge, he offers blueprints and timetables, and he even enters Clipton's hospital hut in search of more workers, and marches out at the head of a column of the sick and the lame. On the night before the first train crossing, he hammers into place a plaque boasting that the bridge was ‘designed and built by soldiers of the British army’.
It is Clipton who asks him, if they might not be accused of aiding the enemy. Not at all, Guinness replies: ‘One day the war will be over, and I hope the people who use this bridge in years to come will remember how it was built, and who built it’. A pleasant sentiment, but in the meantime the bridge will be used to advance the war against the Allies. Nicholson is so proud of the bridge that he essentially forgets about the war.
Lean handles the climax with precision and suspense. There's a nice use of the boots of a sentry on the bridge, sending hollow reverberations down to the men wiring the bridge with plastic explosives. Meanwhile, the British celebrate completion of the bridge with an improbable musical revue that doesn't reflect what is known about the brutal conditions of the POW camps.
Although the film's two most important characters both begin to lose their grip on reality and perspective, the hero more than the villain, we're not quite certain what is intended by that final dialogue.