I'm still talking about problem characters who tend to pop up in your writing and make your story more difficult. This week: the love interest. This isn't so much a problem in the romance genre because the structure of those stories dictates a hero and heroine as co-protagonists (or sometimes protagonist and antagonist) rather than a hero (or heroine) and love interest. It's more of a problem in other genres, where there is a hero doing other stuff, and he may have a love interest waiting at home for him, he may fall in love along the way, or he may be rewarded for success by getting romantic love. I'm going to mostly use "hero" here because this is primarily a problem with stories about men being "rewarded" with women. It can apply to either sex, but I haven't run across too many books with female heroines and non-entity love interests.
This character is mostly a problem when she's treated as nothing more than a motivation or a prize. You could replace her with a valuable vase without changing the plot all that much. I see two key issues that come up with these kinds of characters. If the love interest is already in a relationship with the hero, her being weak and undeveloped makes readers turn against her and want her to die so the hero can be with someone more interesting -- and quite frequently she does, as this character's role is often to be killed so the hero will be motivated to go after the villain. If the love interest is a kind of reward for the hero for his heroism and she's weak and undeveloped, the relationship isn't very believable. We've just watched this hero go through all kinds of stuff, being smart and brave, and then he ends up with this insipid mannequin.
One way to avoid this problem is to stop thinking in terms of "love interest." Unless it's critical to your plot that your hero fall in love with this particular character, just create a cast of characters and see what develops. Give each character a role in the story (aside from love interest), then throw them all together in the plot. You may find one of them fitting well with the hero, and then you can work on developing that relationship. If the plot does require the pairing, give the love interest the same kind of development you'd give to any other character, and find some additional role in the story for this character to play -- friend, partner, member of the team, minor antagonist, etc.
Think twice if your plot requires the hero's wife/lover/girlfriend to be killed in order to motivate him or raise the stakes. That's a trope that's become known as "fridging." Is there some other way other than killing a woman to motivate the hero? If you absolutely must do it, at least let her be a real character who has some other existence than to die for the sake of the plot. Don't just make the hero care what happens to her. Make readers care. Make them feel her loss rather than celebrate because he's now free to be with the far more interesting sidekick.
The same thing applies to a love interest as a reward. Give this character an active role in the story, not just wringing her hands while the hero's in danger and then giving him the big "my hero" treatment at the end. Ideally, readers should be wanting these two people to get together because they like both of them and can see how they'd be good for each other.
Generally, a fictional love story requires some reason the two characters should be together -- affinity, common goals or interests -- and something to overcome -- internal issues, external circumstances. In a romance novel, the conflict to be overcome is the critical part of the story, but outside that genre it may be less important. You don't have to work to keep the characters apart until the happy ending, but you may have a more interesting story if they do have something to overcome, whether it's internal wounds that need to be healed or circumstances that have to be resolved before they have time to start a relationship.
Letting your hero fall in love with a fully fleshed-out character rather than a cardboard standup will only improve your story.