Have you ever noticed that when people are horsing around imitating an accent: Italain, Swedish, Japanese, Russian, Spanish, Chinese – you’re heard it many times – they often produce sounds in combination that are not normally in their inventory of sounds. Yet when you offer them a word in the language to pronounce, they suddenly get tongue-tied – the trilled r goes, the Russian or German ‘ach’ sound goes, etc.
The reason for that is the brain in the first instance is channeling language while in the second the intellectual faculties are engaged, thereby cutting off the brain’s natural propensity to ……….. imitate.