随意 (suí yì) means 'at will, as one wishes' and is a neutral adjective/adverb describing doing something freely without constraint. It contrasts with 随便 (suí biàn), which is more colloquial and often implies being casual or not particular; 主动 (zhǔ dòng) emphasizes taking initiative proactively rather than just acting freely; 自愿 (zì yuàn) is a verb meaning 'to volunteer willingly' with a stronger sense of personal choice.