Not a professional, but…
Manual focus and exposure. On a sunny day, there are usually only two settings to worry about…sun & open shade. If you are walking in a sunny area but notice that you will be entering a shady area, just set the exposure accordingly, and vice versa. Sunny days with fast moving clouds is a bit of trick, but manageable. Overcast day, one setting.
For focus, pre-set or use broad depth of field.
Have exposure and focus set before even lifting camera to the eye…likewise, while lifting camera to eye, think of composition, allowing camera to be up at the eye for less than a second.
Anticipate...and luck...the two big things.
As for legality, it's important to note that different countries have different laws.