I am trying to develop a working theory about the differences between a natal placement of a planet in a sign(say, Venus in Aries) and the effects of a transiting Venus in Aries. it's not that the energies or the potentials for expression of the energies are dissimilar. if they were, astrology would be both unreliable and inconsistent. there are, however, significant variations between these two "conditions" in how they manifest in one's life and that is what I'm attempting to understand. additional complexity arises when you add aspects (contacts between other planets, either natally, or transiting) or the houses in which the planets lay.
the first issue, of course, is duration: a natal planet is in that sign and house in perpetuity (we're going to ignore progressions for now) while a transiting planet is moving through an aspect/sign/house for a knowable length of time.
this time element serves to identify one aspect of the differences: natal planets suggest long term areas of evolutionary potential that a native is working on in this life. the issues/conflicts/growth spurts/crises will predominate in the areas of life indicated by house, the energy and motivation the native brings to these events are colored by the planet, and the sign provides the characteristics of the native's mode of expression: e.g., Venus in Aries in the 6th house could describe a native who is an advocate for women in the workplace, a health worker who does couples counseling, etc. issues of relationships, work, justice and dedication are hallmarks of this natal placement, along with money, conflict and perfectionism.
meanwhile transiting planets create "windows of opportunity" that bring the energy and the motivation of a specific planet along with a particular mode of expression, but are not limited to a particular area of one's life. thus, that Venus in Aries energy - fighting for what you value, the attraction-repulsion in relationships, may end up in the 10th (career) or 4th (family) or 12th (what you need to let go of).
part of what this brings is that even though you were born with Venus in Aries (or whatever), Venus will go through all the signs repeatedly during a normal lifespan, and thus you will have many opportunities to express your venusian energies and motivations in all the different modes (signs) in many areas of your life. this is one of the 'evolutionary' aspects of astrology; there are opportunities for growth and development beyond what is expressed in your birth chart.
Part II: inner planet vs. outer planet transits.