That is true. Interest is all that matters as not many members will feel at home with becoming staff. Some will want to have fun and go out of the site whenever they feel like without having a job that they are doing. So, being sure that they are ready to work is an important aspect of it that you should put into considerations.
Yes, there's a reason why you have to choose your staff from those who indicated interest to join your staff team. It will be wrong to chat up any random member in your forum and give them the opportunity to become your staff. They will never take it serious.