Much recent work argues that lexical frequency plays a central explanatory role in linguistic theory, but the status, predicted effects, and methodological treatment of frequency are controversial, especially so in the less-investigated area of syntactic variation. This article addresses these issues in a case study of lexical frequency effects on variable subject personal pronoun (SPP) expression in Spanish. Prior studies of Spanish SPP use revealed significant constraints including formal and semantic properties of the verb, and discourse factors such as a switch reference. These constraints appear to be confirmed in our analysis of 4,916 verbs from a spoken corpus of Spanish, as is a powerful role for lexical frequency. But the frequency effect--best configured as a discrete rather than continuous variable--is complex; statistically, it has no independent direct effect, but operates entirely through interaction with other constraints. All other constraints on SPP use are amplified in high-frequency forms, and some disappear at low frequencies. Frequency thus acts as a 'gatekeeper' and potentiator: above some frequency threshold, significant linguistic constraints on SPP use emerge; below the threshold they do not. We propose that this reflects experience and acquisition: speakers cannot formulate hypotheses about individual lexical items until they have sufficient evidence; the threshold is the level at which speakers have enough experience with a form to do so. These results have important theoretical and methodological implications. They require rich lexical representations incorporating frequency and collocational information. Methodologically they indicate the need for careful quantitative explorations of frequency, because its role as an enabler of other constraints produces unstable statistical results.