I think one of the, arguably, most important things we as authors strive for when we write is having our characters connect with readers. You can write a palatable story with a strong plot, beautiful prose, and unique characters, but I don't think it will have that extra "oomph," that thing that really makes a story good, if readers can't identify a character, can't find a character who they understand as if they were friends.
Having this connection means the reader has a reason to keep reading, someone to root for, someone they really care about. This connection, I feel, is one of the main reasons people actually read a book.
So, the question becomes how to we make and develop that connection?
I know, there are probably at least a dozen ways. At least! But I think an important one, and one that can sometimes be over looked is humor.
When I'm talking about humor, I'm not talking about Three Stooges slapstick humor, but rather every day humor. The joke you may tell to your friend, or the puns, or the dry sarcasm of a ridiculous incident. The best kind, or the kind I find, is when something ridiculous happens in the plot and the characters themselves makes a joke or a comment about how ridiculous it actually is.
You see, characters aren't isolated from events, and just like people find humor and make witty comments in certain situations, so should characters. Even if it's just one line, humor has a powerful way of making even the most inhuman character human and relatable.
This can be doubly important in Scifi and Fantasy when you're in a strange world, or dealing with strange races. Humor is universal and suddenly makes everything seem not so strange.
So, I guess the moral of all this is, even in the most serious of times and in the most serious of novels, make your character joke. I promise it won't do any harm.