First, I cut out people from magazines. I avoided celebrities or well-know figures so that students didn't have any preconceived ideas about what his/her character would be like.
To save yourself time, you could have students do the prep work for this activity by asking one class to cut out characters for another class. If you only have one class, allow students to cut out the characters for other students.
Then put several characters into an envelope, filling one envelope per student. The surprise factor is part of the fun of this activity. Students will select two people from their envelope to develop into characters, so put at least three or four people in each envelope to give students some choices. I tried to include a mix of genders, races, ages, etc.
After students have selected two people, they will complete a character sketch for each, deciding the details of what his/her characters look like, act like, their interests, etc. Have students glue their pictures of their people on the back of their character sketch in case students don't finish that day or for later reference.
Once students had a detailed character sketch, I presented them with Mad Lib-esque prompts. Students had to insert the characters they developed into these situations and begin writing short vignettes. I created a series of four prompts, giving students about five minutes to start writing in response to each.
It wasn't important to me that students write out a complete narrative for each prompt, I just wanted them to practice crafting a story around their character. You could allow students to pick one or more of the prompts to develop into a larger narrative or have students save their characters for their next narrative writing assignment.
You can find this creative writing activity and others here.
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