Choosing a particular spot is essential. This will determine the size and theme of the painting and if people interact with the artwork or not. It is possible that the visualization of the painting is what leads the artist in selecting a particular space, turning it into a canvas.
Will the wall you choose get the desired attention? If the street side is too busy the artworks message, may go unnoticed. However, if placed in slightly more open spaces like a parking lot, near a local park, hang out spots in smaller streets, people might notice it, enjoy it and the space might turn into a land mark.
What sort of a wall are you painting on? Can you make use of its existing qualities?
Textures can be created or used in different ways. For instance the deep maroon shade above is actually red colour painted over a rusted metal sheet. Wall cracks can be used innovatively. This grey wall with leak marks all over it would make a nice setting for an image of a little girl holding an umbrella.
If you can draw from the type of surface that has to be painted upon, use its texture within the artwork. Or absorb the walls original colour into the illustration it generates a connection between the space and the artwork. However, this can depend upon the creator. Before beginning, it is helpful if the artwork is sketched out with a pencil across the surface. Some artists like working with worn out walls as it sets a totally different mood for their artwork, for such walls spray cans are preferred over paint; while other artists prefer working on smooth outdoor walls.
Having permission to paint on the outdoor space is preferred so that you don't run into any legal issues. However, it helps to carry your sketchbook or copy of work for people or the wall owner to see.