My characters often appear fully formed in my head but I like to try and find an actor or model who looks similar as that helps to cement them in my mind. Pinterest is a great place for this.
As the characters and story build so does the associated board. By the time the novel is written in first draft the board is swamped with pictures some of which I now have no idea what triggered me adding them - which is sometimes the case when ideas no longer fit.
At the end of the writing process, somewhere in the editing, I edit my Pinterest board so it is whittled down to a manageable number of relevant pictures. It's a great tool and also helpful when I'm trying to explain to my publisher what characters look like and can help to inform the cover design.
But writer beware - Pinterest is also a massive rabbit hole where you can get sucked in for hours rather than actually writing the book!
Here's the one I did for Meet Me At Pebble Beach - link.