As one of Hollywood's greatest-ever leading men, Harrison Ford is always in high demand. He doesn't have the time to take every part that's offered to him, and so he picks and chooses what he likes the sound of. Because of his busy schedule, he's said 'no' to James Caan's part in 'Misery,' the title role in 'JFK,' and a leading role in 'Alien.' He has no regrets about passing on them, though.
The role that Ford wishes he could go back in time and agree to is in a smaller, lesser-known film - Syriana. When Ford was sent the script, it was at the first or second draft stage, and he didn't like the sound of it. George Clooney stepped into the breach and played the part of Bob Barnes instead. Ford happened to catch the film, and found that the script had improved significantly since he'd last seen it. He loved the movie, and was slightly envious of Clooney!