Final answer:
Professor Plum is discussing a hypothetical scenario about the killer's location, reflecting a typical element of mystery in theatrical dialogue.
Step-by-step explanation:
The statement "But what if the killer is outside?" by Professor Plum presents a consideration of a possibility rather than a definitive accusation, confirmation, or dismissal. Therefore, the appropriate description of Professor Plum's action is to say that he discusses the killer. Plum raises a hypothetical scenario which suggests a need to account for possibilities that have not yet been considered by others in the room. This line could be a part of theatrical dialogue where characters are trying to solve a mystery, similar to the dialogue found in the provided reference. The reference material suggests a thematic concern with the elements of mystery, the examination of evidence, and the implication of individuals in a crime, similar to what we infer from Professor Plum's statement about the killer potentially being outside.