In my novel, STAR-CROSSED, my hero had no one but the heroine to talk to in the first part of the novel. To cover topics he wouldn’t discuss with her, I didn't want lots of internal monologue or flashbacks which tend to be boring.
What I ended up doing was letting him have imaginary conversations with his best friend. Since he was also stuck in one place, I put these conversations at interesting locations from their shared past that showed more about the hero and his past.
The first conversation, for example, was in a bar on a Wild West style planet where the two friends have rescued a sweet young thing during a bar fight. The two characters shared a beer, talked a bit about the good ol' days, and the hero spilled his guts about what was bothering him.
At other times, the best friend was the devil's advocate for one side of a choice that the hero was trying to make.
If you do something like this, it needn't be as elaborate as an entire scene. It could just be the mental presence of someone whose opinion the character either values or can't escape. Most of us, for example, can hear our mom or dad in our head reminding us to do or not do something.
I’ve also had a character talk things out aloud to a horse he was grooming or a cat she was stroking. The animal’s actions, as if commenting with a purr, a snort, or the shake of the head, gave a nice light touch as well as making the scene more interesting than internal dialogue.
If you want the hero himself as the other character, you should choose some aspect of him you want to emphasize. Say Dr. Indiana Jones--the scholar versus Indiana Jones--the adventurer.
Set up the use of the mental dialogue/scene fairly early in the novel or story so that the important scene when the character finally must make the big decision won't make the reader go "huh?" when the other side of his personality or an imaginary character shows up to discuss the matter.
In other words, have the mental character show up a few times so the hero can tell his other side to shut up or whatever.