This is called fill flash. Fill flash is particularly useful for lightening shadows on faces. As a bonus, when you're using fill flash with people, you can turn your subjects away from the sun so that they don't have to squint. The rim lighting created by the sun in backlit portraits also gives a pretty glow to hair and helps separate your subjects from distracting surroundings. For best results, use a lower medium-speed film so that the camera can provide a good balance between sunlight and flash exposure.
Films are all good choices.
The ideal fill-flash exposure gives your subject about one stop less exposure than the background receives. This opens the shadows sufficiently to reveal detail as it maintains a natural appearance. If you give the main subject the same exposure as the background, the picture tends to look artificially lit. You can create fill flash with dedicated, automatic, or manual flash units. An automatic flash unit will be much easier to use, however, if you have a choice of several flash modes to choose from.