A voluntary interview is not the soft option it can appear to be. The same caution applies, and what you say can be used in exactly the same way as if you had been arrested.
Crucially, you are entitled to independent legal advice before and during a voluntary interview — and that advice is your right regardless of whether you attend by invitation or under arrest.
Having an accredited representative attend with you levels the playing field and ensures you do not inadvertently prejudice your position.