This paper explores the added value of studying intra- and inter-speaker variation in grammaticalisation based on idiolect corpora. It analyses the usage patterns of the English let alone construction in a self-compiled William Faulkner corpus against the backdrop of aggregated community data. Vast individual differences (early Faulkner vs. late Faulkner vs. peers) in frequencies of use are observed, and these frequency differences correlate with different degrees of grammaticalisation as measured in terms of host-class and syntactic context expansion. The corpus findings (...) inform general issues in current cognitive-functional research, such as the from-corpus-to-cognition issue and the cause/consequence issue of frequency. They lend support to the usage-based view of grammaticalisation as a lifelong, frequency-sensitive process of cognitive automation. To substantiate this view, this paper proposes a self-feeding cycle of constructional generalisation that is driven by the interplay of frequency, entrenchment, partial sanction and habituation. (shrink)