Much of the research investigating the perception of speaker certainty has relied on either attempting to elicit prosodic features in read speech, or artificial manipulation of recorded audio. Our novel method of controlling prosody in synthesized spontaneous speech provides a powerful tool for studying speech perception and can provide better insight into the interacting effects of prosodic features on perception while also paving the way for conversational systems which are more effectively able to engage in and respond to social behaviors. Here we have used this method to examine the combined impact of filled pause location, speech rate and f0 on the perception of speaker confidence. We found an additive effect of all three features. The most confident-sounding utterances had no filler, low f0 and high speech rate, while the least confident-sounding utterances had a medial filled pause, high f0 and low speech rate. Insertion of filled pauses had the strongest influence, but pitch and speaking rate could be used to more finely control the uncertainty cues in spontaneous speech synthesis.