We need to use our intelligence, in being focused and alert during chanting.
Chanting of the holy name of the Lord is a serious engagement.
Draupadi called out in all helplessness, and Lord appeared. So when the mood of the chanter is like Draupadi, then Lord definitely appears.
In advanced stage, one could see the form of the Lord right in front of their eyes as a result of chanting.
When you are chanting "Hare Krishna" the body may be breaking, the mind may be reeling but you should know for sure that the soul is in 'ecstacy'.
The quality of chanting dpends on purity. Purity is dependent upon surrender. Chanting and Surrender happen simultaneously.