This paper deals with the factors characterizing the production of autonomous vocalic filled pauses in large spontaneous speech corpora, namely language, gender, speaking style and language proficiency. Two types of corpora are analyzed: a corpus of broadcast news in French and American English and a corpus of short talks in a conference in English spoken by native and non-native speakers. Several acoustic and prosodic parameters are evaluated and correlated with each factor, namely timbre, pitch, duration and density. Results presented here show that the timbre is correlated with language and language proficiency, whereas the duration is linked both to gender and speaking style, the latter conditioning also the hesitation density in speech.