While there is overall consensus that narratives obtained by means of visual stimuli contain less natural language than free narratives, it has been less clear how the naturalness of a narrative can be measured in a crosslinguistically meaningful way. Here this question is addressed by studying the differences between free narratives and narratives elicited using the Frog story in two languages of eastern Indonesia, Alorese (Austronesian) and Teiwa (Papuan). Both these languages are not commonly written, and belong to families that are typologically distinct. We compare eight speakers telling free narratives and Frog stories, investigating the lexical density (noun-pronoun ratio, noun-clause ratio, noun-verb ratio), narrative style (the use of direct speech reports and tail-head linkage), as well as speech rate. We find significant differences between free and prompted narratives along these three dimensions, and suggest that they can be used to measure the naturalness of speech in oral narratives more generally.