It is known that acoustic variation is influenced by the predictability of words and the information that they represent. What is unknown is whether acoustic reduction is also influenced by the referential predictability of thematic roles. We tested this question in two production experiments, where speakers heard a sentence with goal/source arguments, e.g., “Lady Mannerly [source] gave a painting to Sir Barnes [goal],” and described a picture of a subsequent action, e.g., “Sir Barnes threw it in the closet.” We analyzed the duration of full NP descriptions used to refer to the pictured character. We found that duration was shorter for references to the goal than the source, but only in Experiment 2, where the timing of the stimuli encouraged the participant to plan their response incrementally, and not Experiment 1, where participants could pre-plan their responses. The strongest finding across both experiments was that response latency predicted duration, and latency was influenced by the predictability of thematic roles: Goal continuations had significantly shorter latencies. Together, these findings suggest that thematic role predictability does affect acoustic duration, and may be related to the time needed for utterance planning.