I appreciate everybody's input.
Looking at the range maps, the Hammonds doesn't seem as likely. I'm in Los Angeles County, and it's more in line with the range of the Dusky.
I have many other images I haven't shown, and in none of them does the tail appear that long, which appears to be characteristic of the Dusky. So I don't think it's a dusky, either. However I'm no expert.
That said, in none of the pictures is there a white dash in front of the eye, leading to the beak, as there is in the Dusky. There is also mention of a "long beak," light underneath, with "a black tip." I have images which show the underside of the beak, there is no black tip, nor is the beak long. It is light -colored, though.
Habitat and range-wise, and thanks to everyone's input, I'm beginning to believe this is a Pacific-Slope Flycatcher. The range maps match perfectly with where I was, as does the habitat description:
These birds are always in shaded areas, in the lower canyon, always by the San Dimas Canyon Creek. They seem to prefer a specific "shady oak" habitat.
Also, the bill is shorter as well as orange underneath. Here is a different photograph of a different example bird, taken on a different day, much different lighting, but in the identical area and same habitat.
I will try to post more pictures later this afternoon or evening.