Think about the plot of your most recent novel, or work in progress. If you had to summarize the plot, at any point in your recap, would you find yourself saying the word, "coincidentally"? Or the phrase, "it just so happens that..." If so, there's something wrong with your plot, your characters, or both.
I was recently with a young friend who was watching the movie, Zookeeper. In the beginning of the movie, the somewhat shlubby but kind and sincere protagonist asks his super hot, super shallow girlfriend to marry him, in a carefully orchestrated, horseback-riding-on-a-beach at sunset scenario. She not only turns him down, but tells him she'd actually been intending to break up with him because he's just a zookeeper and she can't accept it. Apparently she wants a more worldly and wealthy guy. At this point, I tuned out for a while to focus on something else.
My other task done, I came back to the movie, where a wedding reception was in progress. Shlubby guy was there with his smart, gorgeous co-worker. Hey, do you suppose he'll end up realizing she's a better match for him than the super hot, super shallow girl at some point before the end? I could write a whole different post on predictable retreads of tired rom-com cliches, but that's not the topic for today.
I asked who was getting married, and my young friend explained it was the shlubby guy's brother. Suddenly, the super hot, super shallow girlfriend was doing an elaborate dance with some other guy at the reception. I asked what she was doing at the shlubby guy's brother's wedding. My young friend explained that the super hot girlfriend was one of the bride's closest friends, so she was invited to the wedding and came with her new fiancee.
"So," I asked, "it just so happens that the super hot girl who dumped this guy in the first scene was one of his brother's fiancee's best friends? Isn't that kind of a HUGE coincidence?" She replied, "Yeah, you just have to go with it."
Actually, you don't. And neither do your readers. It was obvious not only to me, but to an 11 year old girl, that the only reason the super hot girlfriend was a friend of the bride was so that she'd be in the wedding reception scene, making shlubby guy jealous and prompting him to his next ill-advised round of hijinks intended to win her back.
It's possible that I missed a flashback in which it was shown how the brothers began dating these besties, but even if there was, it would be very tacked-on and serve only as an excuse to get the super hot ex to the wedding---where of course, there were lots of wacky, slapstick physical comedy set pieces.
Wouldn't it have made much more sense to have shlubby guy run into the ex and her new man somewhere in public, or at a gathering in the home of a mutual friend? After all, if they met at some point in the past they should run in similar circles, or still have one or two friends in common. Of course, this wouldn't have allowed for the presence of the giant ice sculpture and aerialist equipment that played crucial roles in the shlubby guy's public humiliation, but those also had "coincidentally" written all over them. Seriously, who hires a Cirque du Soleil -type aerialist to perform at a wedding reception?!
If the only reason a character DOES something, or IS something, is to set up a later scene, the writer is sacrificing plot and character integrity for the sake of his own convenience, and straining the reader's credulity.
Some might say that Zookeeper also features talking animals, and therefore it's asking too much of the writers to expect much in the way of plot or character integrity. But look at the movies The Golden Compass, Stuart Little, Ratatouille, Cars and Finding Nemo. Talking animals, cars and fish DIDN'T strain credulity in the least in these films, and it's specifically because the writers paid very close attention to plot and character integrity. It's possible to be fantastical and comic, and even a little slapstick, without resorting to the "coincidentallys". If anything, the harder it is for your characters to get where they need to be, the richer your plot (and characters) will be by the end of the tale.