This film was made just one year before the first Indiana Jones film so Harrison Ford was just one year from pure super stardom. This film he remains still as almost the supporting lead of the film, the action hero who technically is the side character action hero. Where Luke Skywalker (Mark Hamill) is the noble hero of the trilogy with his personal journey being to stay on the righteous moral path while fighting evil, Han Solo is the guy who would shoot first in much the same manner as Clint Eastwood's Man with no Name.
Harrison Ford's style as an action hero is pretty much perfect for how Han Solo should be in the film. Han Solo is of course a bad ass who take his enemies down without a second thought, but there is a heart in the guy as well who has a conscience even he perhaps would rather not have one at times. Ford is perfect for this because in his best action performances there is usually a certain lighter humorous touch in his portrayal. In the best way though Ford never seems uninterested in what is going on around him as he is able to make a one liner well still making the situation serious in the same scene.
Ford's just great at being the cool action hero Solo should be and has some of his best moments here. Something that Ford does particularly well is making Solo very competant while sorta being incompetant at the same time. He is very good in showing the imperfect nature of the hero as well as really the effort he has to put in to accomplish his goals. Solo fails plenty of times and he uses these moments particularly well to make Han actually extremely likable but at the same time these moments of failure never compromises his awesomeness as a hero either.
That awesomeness really does come to the way that Ford makes him a man who will always try to do things his way which even includes the romance with Leia (Carrie Fisher). I think Ford's very best moment in the film was the one that was his own ad lib which is the classic last moment expression of love which Han's only response to Leia's "I love you" is "I Know". It is pretty much the summation of the attitude Ford's espouses throughout the film which is a true individualistic streak who always will do things his own way.
This is a very good performance by Harrison Ford and the best of his three performances in the Star Wars film. I would not put it quite as his very best performance in this sort of role, that would come when he became the undisputed lead of a film, but this is a great example of this type of performance from Ford nevertheless. He has just a natural likability as well as believability that succeeds in making a great character to follow through this large scale sci fi adventure, and Ford makes it fully understandable why Solo became such a popular character.