Sri Sri Ravi Shankar: Mithyachar is when one keeps thinking of doing something in the mind but does not act upon it.
For example, you keep thinking about food but you do not eat food.
Mithyachar is also, when you think something else and do something else.
For example, you tell somebody, 'I will be there at six in the evening', even though you have already made up your mind that you won't go, this is Mithyachar.
Lying or fooling oneself is Mithyachar.