Rebbe Nachman teaches us:
Know! a person of truth is one who performs all the mitzvot in all their fine detail between oneself and G-d, just as one does when in the presence of other people. It makes no difference whatsoever if alone or not, since this person has no trace of insincerity which would lead one, G-d forbid, to make some sort of gesture or movement for other people to see. Such a person of truth draws down the power of all the charity that is given, and thereby nullifies the heretical thoughts.
Based on Likutei Moharan I: 251