dialog: You don't have to be good all the time, only when required
 

Thank you for your willingness to provide an answer to my question. I am not remember well the phrase (neither the story of the movie) so please handle my question without "sticking in details".

The movie tells a story in the epoch of Middle Ages.

The metalsmith (craftsman constructing weapons etc. out of metal) of the castle was died and the Master asks his young assistant to take his place. The young assistant has very few experience and hesitates to undertake the job. To encourage him, the Master says something like: "You don't have to be good all the time, only when it is required".

I like very much the meaning of the above phrase yet, I cannot remember the movie. Does some know about ?