I wouldn't count on it since character meals are very popular and usually fully booked. You can keep checking online to see if a cancellation comes up. You can keep checking even while you're at DW since people will cancel up to the day before to avoid the no show penalty. You can also subscribe to the cancellation thread for the month you will be going, but be sure to read the rules in the first post:
http://www.disboards.com/threads/new-cancel-thread-guidelines-march-september-links-here.1802381/
Walking up would be a last resort unless you're staying at the CR because it will take time to get to the restaurant without knowing whether you'll be seated. If you have young children this could a be disappointment if they get all the way there and then find out they won't be able to meet the characters. If you want to try walking up, most suggest getting there at opening.