Is it okay if the inciting incident is a coincidence?
Question: Can what sets everything in motion (on the first page) be something that 'just happened', or will that disappoint readers?
Yes. A coincidence that draws the main character into the plot is a device often used in fiction, particularly if your main character is an unlikely hero.
Unlikely heroes (or heroines) are characters who are ordinary people with no extraordinary training or talents who find themselves caught up in extraordinary events, often just because they were in the wrong place at the wrong time.
A few examples...A Hitchhiker's Guide to the Galaxy
: Arthur Dent, an ordinary guy gets caught up in a journey across space and time when his best friend turns out to be an alien.The Lion, the Witch, and the Wardrobe
: a group of children are sent out of London during WWII and just happen to be billeted in a house with a magic wardrobe that leads to a magical kingdom.
The film Dave
: ordinary guy happens to look so much like the President of the Unites States that he is hired to stand in for him and ends up becoming the President.
The film Notting Hill
: the most famous actress in the world just happens to walk into a book shop and changes the life of the owner forever.
These coincidences don't just happen in science fiction, fantasy, thrillers, or other books involving extraordinary events. Any story can begin with a coincidence and ring true because in real life coincidences happen all the time that lead people in unexpected directions.
You happen to meet someone at a party. You stumble across some information. Your school hires a guest speaker. You read an article in the dentist's office. New neighbours move in next door. These little events can change everything.
Of course, what works for an inciting incident doesn't work so well at the climax. Readers want to see how a character's choices determine the outcome. They want to see what the character does with the opportunities and challenges that come his way.