Real-time sentence processing involves connecting linguistic input with knowledge. Here, we ask how variability in semantic memory (specific domain knowledge) may influence semantic access in real-time sentence processing. We recorded EEG while participants more/less knowledgeable about the narrative world of Harry Potter (HP) read sentences. In Experiment 1, all participants showed N400 predictability effects for general-knowledge sentences, but only those with high HP knowledge showed predictability effects for sentences about Harry Potter. This effect was driven by graded brain responses to predictable endings as a function of knowledge. Experiment 2 revealed greater semantic activation (inferred from N400 effects) for HP items participants reported knowing. High-knowledge participants also showed greater semantic activation for items they reported not knowing/remembering. These findings suggest that amount and/or functional organization of knowledge has real-time consequences on written sentence processing and implicate implicit/partial access to domain knowledge for experts when information is not explicitly recalled.