There are a variety of answers to your question, depending on which characters your asking about. It almost seems like the trial is a spectacle now, with the way that people from all over the town and county make it to the courthouse to see the happenings. Many people pass by the Finch house on the way to the courthouse, and Jem comments on them, telling Dill and Scout about each person and the gossip he knows.
When the children sneak to the courthouse, they find it packed with people. It is here that Scout learned from the Idlers' Club (a group of men who hang around the courthouse) that Atticus was assigned the trial--he didn't choose to take it.
Because the main level of the courthouse is so full, Scout, Jem and Dill sit with the black people in the balcony. The audience of the trial acts just like the audience in a movie or at a play--they laugh, "ooh and aah", and there is even an outburst from Link Deas. For all of its seriousness, the trial is almost portrayed as a theatrical event, which is probably was to some of the people in attendance.
In the small town of Maycomb, Tom Robinson's trial becomes a spectacle that attracts the entire community. Throughout the trial, the audience is attentive and relatively respectful during the hearing. Scout mentions that women quietly shifted their babies while some of the African American citizens whispered softly to each another. Jem quietly comments on certain aspects of the hearing and is intrigued the entire time. At the beginning of the trial, Scout says that all the spectators were calm, except Jem. However, certain moments excite the crowd, and Judge Taylor has to hammer his gavel to calm them down. After Judge Taylor threatens to hold each spectator in contempt of court, the audience immediately becomes quiet. Scout also mentions that during the recess the spectators did not move and stayed in their seats, which was unusual in Maycomb's court. After Atticus questions Tom Robinson, Link Deas gets kicked out of the courtroom for interrupting the proceedings to defend Tom Robinson's character. Dill also creates a disturbance by crying after he witnesses how Mr. Gilmer speaks to Tom Robinson. Before Judge Taylor reads the verdict, Scout says,
"Sometimes a baby would cry out fretfully, and a child would scurry out, but the grown people sat as if they were in church. In the balcony, the Negroes sat and stood around us with biblical patience" (Lee, 128).
Except for a few minor disturbances, the audience is relatively quiet and attentive throughout the trial.