I think that Greene builds up suspense in two separate ways. The first way is through the intrinsic drama of the legal proceedings. Repeatedly, the narrator suggests that the trial itself is not a "big deal." It is seen as something almost a matter of fact. In this, Greene builds up suspense through the moment in which the trial turns into something "extraordinary." The cross examination of Mrs. Salmon is a moment where suspense is built and suddenly the ground upon which the reader felt was certain is far from it:
Counsel took a look round the court for a moment. Then he said, `Do you mind, Mrs Salmon, examining again the people in court? No, not the prisoner. Stand up, please, Mr Adams,' and there at the back of the court with thick stout body and muscular legs and a pair of bulging eyes, was the exact image of the man in the dock. `Now think very carefully, Mrs Salmon. Can you still swear that the man you saw drop the hammer in Mrs Parker's garden was the prisoner - and not this man, who is his twin brother?' Of course she couldn't. She looked from one to the other and didn't say a word.
I think that this helps to enhance the suspense in the court because it completely shifts the focus of the trial. From something that was framed as more procedural, there is a clear demarcation where uncertainty and doubt has emerged. Once Mrs. Salmon, the supposed bedrock of all witness testimony changes her story, the rest of the witnesses follow suit. In this dramatic moment, suspense is heightened from a legal standpoint.
I think that the second way in which suspense is built up happens outside of the court room. The hustle and bustle outside of the courtroom is filled with an element of suspense. There is the desire to whisk both brothers out the back way of the courthouse, to which one protests by saying he was exonerated. The crowd of people, the chaos still brought on by the seismic shift of the court proceedings, as well as the fact that some element that might seek to embrace action outside of the courtroom all conspire to bring out the dramatic ending:
Then it happened. I don't know how, though I was only six feet away. The crowd moved and somehow one of the twins got pushed on to the road right in front of a bus.
It is here where the suspense reaches its most intense moment in the story. Through this, Greene is able to build up suspense both in the courtroom and outside of it. As a result, there is a great intensity of drama and suspense in the short story as it takes place in two different realms, contributing to an overall suspenseful effect in the reader.