Mostly, it is Potter falling down. She has the ability to do physical comedy, but the nonstop barrage is tiresome. Falling down once is amusing. Falling down multiple times is not. Potter's voice also has a wonderful quaver in it that helps cement her character.
Pierce always seems to fall hopelessly in love with the wrong man.
As soon as she sees Prinze, she swoons. Her roommates scheme to set them up, and when they finally do get together, Pierce has to try to hide her anxiety over his supposed murder. Prinze is not suited for this role. For one, he still looks like he's in high school, and years younger than Potter. His only job in the first half of the movie is to give wide smiles to the camera.