General synonyms:
* Pretend
* Simulate
* Affect
* Assume
* Imitate
* Counterfeit
* Dissemble
* Fabricate
* Fake
* Sham
* Masquerade
More specific synonyms:
* Hypocritically (to feign something while holding a different belief)
* Deceive (to intentionally mislead)
* Misrepresent (to present something falsely)
* Act (to behave in a particular way)
* Pose (to adopt a particular appearance)
The best synonym to use depends on the specific context.