What?? A leading man is a leading man because he carries the film (or is the love interest of the woman who carries the film). He tends to be in his 30s or older, especially in theater, but not always. Younger romantic leads are traditionally termed junior leads, b-couple, or 2nd leads. it has absolutely nothing to do with looks, although in musicals the male romantic lead traditionally is a baritone or baritenor (now probably tenor). Paul Muni, Spencer Tracy, and Humphrey Bogart were all leading men despite not being traditionally handsome (Bogart had a scar and a lisp) like Errol Flynn or Clark Gable. Steve Buscemi is a leading man. So's Adam Sandler. Jim Carey is also a great leading man, even in dramas. Jesse Eisenberg is a leading man as well. Peter Dinklage, despite his stature, is the most traditional-looking leading man of any of the men I've named here. It's got absolutely nothing to do with looks. It's about whether you carry the production (and whether you can do that successfully a consecutive number of times) or whether you're paired as a romantic lead. Nothing more. A leading man is a position, not an archetype. It changes based on the genre and the time period. In the 20s the leading man was exotic and flamboyant. In the 50s he was a gangster. In our time (when it's not in an action movie) he's a slightly dweebish Everyman kinda guy (Joseph Gordon Levitt, Ryan Gosling, Jesse Eisenberg, Seth Rogen). A character actor, because people will ask, is either an actor who looks very strongly like a certain type and fills in for that type (ie. John Goodman, Christopher Walken, Luis Guzman, Tommy Lee Jones) OR an actor who, via makeup/speech/behavior is good at playing a wide range of characters (Meryl Streep, Bette Davis, Johnny Depp, William Hurt, Marlon Brando, etc). Some, like Phillip Seymour Hoffman lie somewhere in between in terms of what kind of character actor they are. Some, like Heath Ledger or Johnny Depp (until recently) or Matthew McConaghey can be a leading man (can carry a film on his own as protagonist or romantic lead) who is also a capable character actor. Edit: John Cusack, guys.
best opinion I read on this topic..
https://www.reddit.com/r/acting/comments/23kf5y/what_qualities_must_a_leading_man_posses_to_be/