Crossword grids have been my Saturday ritual for years, and when that little clue asks for 'protagonist' I almost always eyeball the crossings and think 'HERO' first.
H-E-R-O is the classic, portable fill: short, familiar, fits a ton of themes, and pops up in both easy and Sunday-sized puzzles. But I also watch for context. A capitalized clue or one referencing a specific work will steer me toward a proper name — for example, 'Protagonist in 'Hamlet'' would be 'HAMLET' — while a gendered clue might call for 'HEROINE'. If the grid wants a three-letter fill, 'MC' (short for main character) shows up occasionally, and 'LEAD' or 'STAR' are common four-letter alternatives depending on whether the puzzle emphasizes acting or narrative.
I like to parse the clue voice: cryptic or clever clues might hint at 'antihero' or 'tragic hero' which are longer fills and change letter patterns. My practical tip: pencil in HERO as a starting point, then let crossings prove or disprove it — that little thrill when H-E-R-O slots into place still makes me smile.