Simultaneous interpreting is a complex task where the interpreter is routinely involved in comprehending, translating and producing language at the same time. This study assessed two components that are likely to be major sources of complexity in SI: The simultaneity of comprehension and production, and transformation of the input. Furthermore, within the transformation component, we tried to separate reformulation from language-switching. We compared repeating sentences (shadowing), reformulating sentences in the same language (paraphrasing), and translating sentences (interpreting) of auditorily presented sentences, in a simultaneous and a delayed condition. Output performance and ear-voice span suggest that both the simultaneity of comprehension and production and the transformation component affect performance but that especially the combination of these components results in a marked drop in performance. General lower recall following a simultaneous condition than after a delayed condition suggests that articulation of speech may interfere with memory in SI.