When experts are immersed in behavior, do their brains prioritize cognition and the movements needed to enact choices? Most efforts to understand neural activity during well-learned behaviors like decision-making focus on cognitive computations and the restricted movements needed to receive rewards. We wondered whether animals explore the movement landscape more broadly during decision-making, and how this impacts neural activity. To test this, we characterized movements using video and other sensors and measured neural activity using widefield imaging. Activity cortex-wide was dominated by movements, especially “fidgets” and other uninstructed movements that reflected an unknown priority of the animal. Some uninstructed movements were aligned to trial events. Accounting for them was critical for interpreting trial-averaged activity because neurons with similar average activity often reflected utterly different combinations of cognitive and movement variables. Other movements occurred idiosyncratically. Accounting for them was indispensable for understanding trial-by-trial fluctuations that could have been misinterpreted as “noise”. We replicated these results with single-cell resolution using two-photon imaging and Neuropixels probes in cortical and subcortical areas. Our observations argue that during cognition, a rich movement landscape is explored, with profound consequences for neural activity.