L.A. as a whole is low in pedestrian traffic density, due simply to how sprawling it is. We have over ten million people, but they are spread over a wide geographical area, so they generally do not walk. So, if you want densely populated sidewalks with lots of activity, you have to go to neighborhoods where people conduct business on foot...and these are neighborhoods in which large numbers of people do not own cars...because they cannot afford them and/or because they are immigrants from places where cars are simply not a part of the average family's life.
My two first suggestions if you want a taste of real L.A. (average L.A. people living their actual lives) are to try walking down 6th on the eastern side of downtown any time (the middle of the night is fun ), and to try walking down Cesar Chavez and/or Whittier Boulevards east of the City on a weekend day (Boyle Heights, East Los Angles). Barring these, you might want to hit the streets in the Westlake/MacArthur Park area at afternoon to evening rush hour, or on weekends. 6th, 7th, and 8th are good bets. If you want to shoot some Asian-American enclaves, try strip malls in Alhambra, San Gabriel, and Monterey Park. (Some main drags here are Vally (E/W), Garfield (N/S), Atlantic (N/S, will also lead you into the eastern part of East L.A.), and Garvey (E/W).
If you want to capture the strange desolation of our sidewalks, try almos anywhere except where I mentioned! Seriously, I would try the west side (west of Western technically, but I always say that west of Alvarado is where it really starts feeling like the west side), the San Fernando Valley, or Orange County (insert dramatic creepy music here): The Weirdest Place on Earth.
If you are lazy and unadventurous, Hollywood is OK, just for the fact that it is such a wretched piece of $hit of a place in almost every way. Grand Central Market and other areas of downtown could prove fun and fruitful as well.
Have fun!!!! Don't let the coldness and the emotional and physical distance of the people here intimidate you.
FWIW, I am a sixth-generation Los Angeles person.