Photocomps, when they're done well, look like they're easy. Often the most time-consuming part of the process is gathering photo assets which fit into the concept. Inevitably, I'll find a few "anchor" pieces which I'll be able to build around.
I'll have to piece things together to match the layout as closely as possible, and then will usually have a fair amount of color correction, digital painting to bridge the gaps and filtering to blend everything.
Hopefully, if I've done a good job, the viewer isn't noticing the work done, but only looking at the concept and the image created as believable.