Formal:
* Pretend: To act as if something is true, even though it is not.
* Simulate: To imitate or reproduce something, especially for the purpose of study or testing.
* Affect: To put on a false appearance of; to make believe.
Informal:
* Fake: To create a false impression.
* Make believe: To pretend.
* Act: To behave in a particular way, especially in a way that is not genuine.
* Put on: To adopt a particular attitude or behavior, often in order to deceive someone.
The best synonym to use will depend on the context and the desired tone.