The thing I remember Ford saying about Solo is he felt there just wasn't much going on with the character. I think that's why he sleepwalked through ROTJ.
I imagine they wrote some meat for him to act through to get him to return.
Okay, here's where I pontificate on the character of Han Solo, versus the character of Indiana Jones.
I saw the interview you're referring to. He referenced Han Solo as one-dimensional, and that there wasn't anything to explore there, but when asked if he would like to play Indiana Jones again, he said "in a New York minute."
There's one important thing to keep in mind, here. First is that Indiana Jones is THE main character in those movies. Han Solo is part of an ensemble.
Indiana Jones is an EXTREMELY one-dimensional character. He goes after treasure, he gets in trouble, he gets out of trouble, he overcomes. That's it. It's a WONDERFUL character, and we all love him, but that's the definition of a one-dimensional character, a one-trick pony.
Conversely, the character of Han Solo is RICH with character development.
When we first meet him, he's down on his luck and looking to get out from under his debt to Jabba. He meets Luke and Ben and Leia, and, through self-preservation, is able to overcome. Though conflicted, he takes his reward and sets off to handle his business. During that time, he apparently has guilt over his decision (enhanced by Chewbacca, no doubt), and returns to the fight that he originally called 'suicide.'
He frees Luke up to take the shot that destroyed the Death Star. He was recognized as a hero and gained a place within the Rebel Alliance.
He decided to stick with them, still keeping his options open, but was unable to abandon the Rebels on Hoth due to his attraction to Leia.
He ran with it, which ended in his imprisonment in carbonite, which he was freed from because of the bonds he had established with Luke, Leia and Lando (and of course his bond with Chewbacca). If he had not grown as a person since we met him in ANH, he would have eventually been captured by Jabba, and wouldn't have had the powerful friends that he had to come and rescue him.
Once rescued, he is given proper recognition by the Rebel Alliance and made general, and entrusted with a crucial mission to defeat the second Death Star.
He's conflicted with his love of Leia, but serves nonetheless, only to find that his devotion and love is returned.
This is a character that went from a cold loner to general within a revolutionary army, both depended on and depending on others.
If that isn't an INCREDIBLE character arc, I don't know what is.
I LOVE Indiana Jones, but Han Solo has sooooo much more depth, comparatively.
But Indiana Jones is the STAR of the movie. I think that that's what was most attractive to Harrison Ford.
/rant.
Sorry, I get deep into this ****, lol!