Probably the main thing is to be confident when you think you're right, but humble when you realise you're wrong or in error. Nothing seems more suspicious (or just annoying) than someone who doggedly refuses to back down from their point.

No matter how confident you are in your theory, mistakes will happen (as it self-evident with Theodore's lynch). I personally like it when people can fess up without throwing a tantrum, but then I also like it when everyone gets along great, which simply isn't always possible in Mafia!