'Conversation' type programs are easy to create in Scratch. The program below goes further than a pre-programmed conversation for it is interactive - it invites your input so that no conversation is quite the same as any previous one. Download the program HERE, study the code and try to make your own original conversation, using different images. It would be a good idea to first make a conversation that does NOT include interactivity and then to add interactivity later.
As well as looking at the scripts for the stage and each sprite, be sure to look also at the backgrounds and costumes for that will help you understand how the program works. The most interesting think about this program is that the scripts of the different sprites need to communicate with one another and the only way for them to do that is by way of broadcast messages.
One way to make your own conversation more original and interesting would be to use photographs that you have taken of you and your friends, with a locally-taken photo for the background too...