If someone spends weeks saying and doing nice things, doesn't that make them a nice person? Are kind acts negated by someone "not really being nice"? What does that even mean?
The real vs fake thing seems very naive, that isn't how humans work. Personalities are fluid and change depending on situation, company, mood, stresses, etc. There isn't a "real" person hiding beneath a surface with anyone. E.g. if someone is quieter in large groups, that's just an aspect of their personality, not them hiding anything. Dylan being loud and aggressive when he was upset doesn't mean he's "fake" if he's been a shoulder to cry on some other time.