Climate
It can get very hot in France and in Italy in August. August is also difficult as France is shut and Italy full. If you start North of the Grand St. Bernard Pass bear in mind that the pass is often snowbound between late September and mid June. Take your choice. It can rain any time.
Companions
This can make or mar your experience of the trip so it is worth considering carefully. Even if you have known the person for some time, if you have never walked with them before it is strongly urged that you do some practice walking together and preferable for a number of consecutive days. Many who are interested in the Via Francigena have already done an amount of walking on one or more of the caminos to Santiago. As has been stated elsewhere, infrastructure etc on the Via F is not anything like as well developed as on those routes so it can get quite fraught!
It would also be wise to discuss and agree beforehand how much you are prepared to spend each day, what sort of meals and accommodation you will want, how much ground you wish to cover daily, whether to have rest days and if so how many and where.
Compostelle
see entry for Testimonium under 'T'
Credential
See entry for Pilgrim Record under 'P'