Here are a few examples of midpoint combinations:
J.K. Rowling (she wrote Harry Potter): Mercury conjunct her Sun/Moon
Lewis Carroll (Alice in Wonderland): Sun semi square Moon/Mercury
Martin Luther King: Moon semi square Sun/Mercury
Belgian author Hugo Claus: Moon semi square Sun/Mercury
Maigret: Sun square Moon/Mercury
Agatha Christie: Moon conjunct Sun/Mercury
Jean Auel: Mercury conjunct Sun/Moon
When Mercury is in hard aspect with your Sun/Moon midpoint you will be especially motivated by discussions, writing, speaking and communicating in general. When there is another combination of Sun/Moon/Mercury in your chart, you will communicate with heart and soul.
*) For more about calculating and interpreting midpoints, use this link to Astromarkt, the site with background info for this blog. In short:
The Sun/Moon midpoint is the equavalent of (Sun + Moon)/2. Of course, to calculate this you need to use the degrees of the 360 degree circle. A Sun in the 10th degree of Taurus is at 40 degrees, a Moon in the 20th degree of Libra is in the 200th degree. The midpoint Sun/Moon of this example combination is at 120 degree (zero Leo). If Mercury is in the 1st degree of Taurus there is Sun/Moon square Mercury. (Don't use more than 2 degrees orb). Now that I write this Sun/Moon is almost square Northern Node (at midnight GT). That means that motivation comes from a group of people (family, nation, tribe or community), like 'the people' or 'clients' or 'fans'.
Also see Astromarkt, for example about Sun/Moon and AC/MC midpoint combinations. If you are more interested in Sun/Moon in synastry, go here...