Taken from a very reputable book from the 19th century called The Way of Interior Peace by Fr. Lehen, pg. 99:
“To a mortal sin belong three points: 1st. Weighty matter; – a jesting lie, for example or a vain, self-conceited thought, is not sufficient for a mortal sin. 2d. Full knowledge of the evil; that is, the deliberate consciousness that what one does is a mortal sin. This excludes all cases in which the sin proceeds from surprise, or in which the soul is not full master of her powers; as, for example, in half-slumber, and the like. 3d. Full consent of the will to that which the understanding knows to be mortal sin. As long as the consent remains imperfect, or we are conscious of a certain hesitancy, a deferring, or a reproach of conscience in consequence of our neglect in combating the temptation, the sin is only venial.”
I hope this helps.