It's pretty simple if you do it the straightforward way of editing files. The downside of that of course is that if you want several different mods that edit the same file you're in a bit of a pickle. Dunno how much more difficult it is to make a proper mod that will just modify the part of the file it needs because I haven't tried it, but if you're just making something for yourself the former method should suffice.
http://imgur.com/gallery/WCfeO
http://community.playstarbound.com/threads/mod-list-modding-guides-and-general-modding-discussion.106868/
The first link is handy for unpacking the files, the second one has a bunch of guides for how to actually change stuff. The first guide (Modder's Guide by Alpha_Omegasis) is the only one I've looked at so far, and I didn't look at much besides the photo edit section but there is definitely a section for dialogue. I would imagine there's also a file containing characters' likes and dislikes.
What I'd like to do next is replace the dog with a schipperke and the horse with a husky since I already named them Belka and Strelka respectively, but animated sprites of animals might be a bit outside of what I can manage to pull off. :/