I had a writing teacher tell me that she makes pintrest boards to flesh out her characters and that means pinning pictures of celebrities. I've never done it, but now it sounds fun! does your MC resemble a celebrity? Is their personality similar to an actor/actress in a movie role? Is their style based on someone (for ex. X-File's Dana Skully's character design was based on Clarice Sterling in Silence of the Lambs)? I always liked Jonathan Crombie as Gilbert Blythe from the Anne of Green Gables movies, so when I picture a gentleman, Crombie comes to mind, haha!
The two main characters in my current WIP physically resemble (at least in my mind) Nicholas Hoult and a younger version of Krysten Ritter. Side note, I have a huge crush on both of them. Don't tell anyone.
When I created my protagonist, in my imagination he looked like a combination of 60s and 70s movie tough guys, James Coburn and Lee Marvin transposed to a medieval/fantasy world : When I painted a picture of him using Gimp, he came out looking more like a combination of Hugh Jackman, Mel Gibson and Arnold Schwarzenegger: Personality wise, he's closest to James Coburn with a little of Lee Marvin's mean streak.
A thin version of Dwayne "The Rock" Johnson with hair. That's how I picture Koa, my MC from my, currently on hiatus, WIP. The others don't resemble anyone I can think of.
He's similar to John Wick in personality, but he doesn't particularly look like any celebrity. I can't find a picture of anyone that resembles him. His looks/clothing/style are all roughly based off if various stereotypes.