Why cast Tom Hanks for this role?
Answer is obvious - too many academy awards.
It's a great movie - but it could have been better.
Lets start a poll - who would have been better as Michael Sullivan?
I like Ray Liotta.
Hanks was perfect in this movie. He was sublime. And as a lot of people have already stated, this is a very underrated performance. It probably doesn't get the attention it deserves compared to his other more well-known roles, but it is, nonetheless, just as brilliant as his other roles, in my humble opinion.
Hanks is a beloved actor, and I think that plays a huge part in why Hanks was cast because he would be able to illicit sympathy from the audience fairly easily. That sympathy for Sullivan is absolutely critical because without it, the whole movie would fall apart. The audience has to accept that Sullivan is a good man even though he is a guy who does a lot of bad and nasty things -- he works with gangsters and kills people for a living. A lot of people in the audience would have a very difficult time embracing this type of character. However, I think Hanks was cast perfectly, mainly because he is Tom Hanks. No matter which role he plays, Hanks has an inherent good-guy image that transcends his roles.
I'm having a hard time coming up with anyone who could have played Michael Sullivan any better. In order for the audience to buy into this other actor's portrayal of Michael Sullivan, he'd have to be able to come across as a good man who is forced to do things he is not proud of. One actor whom I thought shared Tom Hanks' inherent good-guy image was Robin Williams, but I'm not sure if Williams had the acting chops to pull off playing a mob hit man.