Synonyms for Spontaneous (same or very similar meaning)
WordNet sense 1 (unthinking; prompted by (or as if by) instinct):
instinctive, natural
WordNet sense 2 (happening or arising without apparent external cause):
self-generated
WordNet sense 3 (said or done without having been planned or written in advance):
ad-lib, unwritten
WordNet sense 4 (not furnished with or using a script):
unscripted
From the ODE community, based on WordNet
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