Was Jenna Fischer actually drunk for the Chili's scenes? Or was she just acting drunk?
She wasn't drunk but she researched it
According to this interview, she wasn't drunk in the scene, and she'd never really been that drunk in real life. But she researched it on how to act being that drunk:
She went out and got some real life experience on the subject. B.J. Novak, who wrote and produced the show while playing Ryan on the series, agreed to be her designated driver for an evening and helped her gage how she was feeling after each drink that she had. "I don't remember a lot after that," she said about her third drink during their little experiment. "But, I researched it. Is that weird?"