Ask your audience to suggest a first line and last line to be used in a scene. This can be done as the audience enters the theatre by having them write lines down on scraps of paper, or can simply be asked right before starting the scene.
Select a starting line and finishing line from the audience suggestions. Repeat your selections to the audience and actors. Foir example, the first line will be, "I want to buy a llama," and the last line will be, "At least my mother isn't dead."
Start the scene by having two actors begin on stage. Obviously, one of them should start the scene by speaking the selected first line.
Have the actors organically work towards the last line. For example, if the first line is about llama sales, and the last line is about mothers, one actor might suggest that llamas are a gift the whole family can use.
Continue the scene until one of the actors quotes the selected last line. The other actor should be setting him up for the line in some way, although this does not have to be rushed.