Characters do not randomly walk around outside. Most are indoors and lines can be long...especially if you want to see Mickey. There are some characters outside, but they will be in specific spots and again you will have to wait in line. (You do a lot of standing in lines at Disney!)
There are photopass photographers in various locations throughout the park and many of these are for pics of landmarks- the Castle for example.
Out of curiosity, are you only there for one day total or will you be in the parks any other days? For a 1st trip, if at all possible MK is easily a two park day.