The chase through the sewers is not just a marvellous combination of acting, directing and cinematography, but an outstanding example of editing. From In Search of the Third Man (Charles Drazin):
A serious difficulty was Orson Welles’ failure to arrive in Vienna until long after he was due. It was decided as far as possible to double for him. Guy Hamilton (Assistant Director) was one of the people who acted as his stand-in. [Those are his fingers clutching the grate at the end.] He became adept at running in front of an arc light and sending a shadow scurrying down alleyway walls. To hide the fact that he wasn’t Orson Welles, he wore a big hat and a black coat, and Reed made him leave a coat-hanger in to bulk out his much slimmer frame.
When Welles finally did turn up…he launched into a tirade, furious that he should be expected to work in such filthy conditions. The British crew…were rather taken aback but there was nothing they could do except watch him storm off. ‘Well,’ said Reed, 'it’s his first day, he’s nervous, and we’ve got so many other things to do. We will just have to build a sewer in Shepperton.’
Given his refusal to go down in the sewers there was really very little for Welles to do in Vienna. He 'had one day’s shot walking to the Prater wheel, and walking away from the Prater wheel,’ recalled Guy Hamilton, 'and I think I’m right in saying that was the sum total of Orson’s Viennese contribution.’ All the shots of him in the sewers took place many weeks later at Shepperton - with its more hygienic water supply.