I don't know about the rest of worldbuilders out there, but I spend a lot of time thinking of my various realms - usually in great detail. I figure if it doesn't feel real to me, it won't feel real to anyone else. This is of course a bit futile because aside from my wife and a few close friends, not many real people get a glimpse of my imaginings. I would post such minutiae here, but it would likely be very much dry exposition. Perhaps someday there will be published volumes, appendices, and sourcebooks. Maybe.
That's what I'm getting at,. Palladium is, I hear, among the most complete worlds ever put on paper. They include not just people, but critters from mundane wildlife to horrific beasties. I'm sure others have done so as well, but Palladium is the only one that yours truly has come across that shows this level of detail. Tolkien did so too, but in a different way - his was more about language, history, and mythology. To a degree, as a fictional world traveler, I'm less concerned with the locals' language, but I do like to know their naming patterns (this is where I typically start) and who/what are their gods. I'm not discounting it altogether, but those are subjects for another post.
What I as a paranoid tourist want to know is: what is going to try to kill me? Is it safe to go out at night? Can I drink the water? What's on the menu? Am I potentially on something else's menu? Overall, what slinks, crawls, slurps, writhes, growls, barks, itches, bites, stings, etc?
When deciding what kind of plants and animals fill up your world, consider the climate and food chain. Are we talking tropical with huge insects and dinosaurs? Is it frozen tundra with woolly mega fauna? Maybe a swampy jungle with all manner of poisonous fangs and thorns and lurking, unseen predatory cats. Does the danger hang from the trees, reach up from the ground, or emerge from murky pools?
To help with your wonderings and wanderings, angelitoon over at deviantart was kind enough to allow me to share some pics. These are (simple) thumbnails of creature concepts. I find that it helps to only have a rough "shadow" of the critters because it really makes the imagination stretch to think of what they'd look like up close in the wild. Enjoy!
Things like this require some imagination and research into botany and zoology. Meteorology and climatology are fields to consider as well. Even if you only look at what flies, strides, or swims on planet Earth, it's all there for a reason. It's generally linked as well through environmental systems from simple to very complex.
If you say your world is "normal" and very similar to that which we already know, look at weather/climate as it's own monster. What threats does it pose? What happens if you're caught out in the elements? Depending on where you're stranded you could freeze, fry or both in a desert scenario. What about natural disasters? Floods, draught, volcanoes, earthquakes, hurricanes, tornadoes, fires, etc. Where do they happen? How often?
When worldbuilding remember, all things considered. Even if you don't intend to work such encyclopedic knowledge into the story, you need to have an answer ready in case someone asks. it's much better to have already thought of it than to get caught unawares with a "gee...I dunno."