How does Wilder use flashback in the motion picture, Witness for the Prosecution?

Much of the story is told through flashbacks. While Leonard tells his story, viewers are actually able to see the events occurring as he describes them. Because we see Miss Emily French and his friendship with her, we, like Robarts, are inclined to believe that he's telling the truth. The flashbacks are deceitful, because they are from the point of view of the person telling the tale.

Flashbacks also help the audience learn more about Christine. Viewers are easily able to see how she and Leonard got together, although we can't understand her cold feelings. Realizing that she was an actress also helps us when the truth comes out and we realize, as Robarts does, that he has been tricked.

The flashbacks bring the events and characters to life. Wilder actually lets the audience see the characters who are now dead. Since we do see Emily French, we feel pity for her and are also more likely to believe Leonard and Christine's lies.