I usually have my "people" camera wearing a hotshoe flash. The flash will even out the exposure somewhat between the subject and the background. It will also open shadows around the eyes and very possibly provide catchlights in those eyes.
I also like to use a longer lens for people photography/ Your 55-250mm lens should be perfectly adequate. The longer lens will blur the background which will help to isolate your subjects from that background.
However, if you want to do an environmental portrait with a shorter lens flash can be a very important facet. I shot this knife sharpener in China using a 35mm focal length on a Canon 30D. He was casting a shadow over his grinder and the cleaver he was sharpening. The flash opened up that shadow...
OTOH, when shooting in an overcast environment without harsh shadows and without a bright background, the flash is not necessary as in this image of a street musician in Xi'an China.
What is necessary, IMO, is to have the face and eyes in focus - even when the eyes are closed as in the case of the musician...