Bhagavad Gita · Sankhya YogaChapter 2, Shloka 62 (2.62)

The ladder of downfall (first part):

1. Dwelling — thinking about a sense object. Shankaracharya: viṣaya-viśeṣān ālocayataḥ — repeatedly looking at and thinking about the particular qualities of objects.
2. Attachment (sanga) — Shankaracharya: āsaktiḥ prītiḥ — attachment, fondness.
3. Desire (kama) — Shankaracharya: tṛṣṇā — thirst. Ramanuja gives a precise definition: kāmo nāma saṅgasya vipāka-daśā — desire is the ripened state of attachment; puruṣo yāṃ daśām āpanno viṣayān abhuktvā sthātuṃ na śaknoti — the state in which a person simply cannot stay without enjoying the object.
4. Anger — Shankaracharya: kutaścit pratihatāt — when desire is blocked for some reason. Ramanuja: the desire is there and the object is not at hand — so anger at the people nearby: ebhiḥ asmad-iṣṭaṃ vihatam — "they have obstructed what I wanted!"

Where does it start? Ramanuja says something serious: one who has not removed attachment to sense objects and has not fixed the mind on the Lord, even if he sits restraining his senses — anādi-pāpa-vāsanayā viṣaya-dhyānam avarjanīyam — because of beginningless tendencies it is hard to stop dwelling on sense objects; and through dwelling, attachment becomes ati-pravriddha (greatly increased).

That is, it begins with a thought — before any action.