Practice Makes Perfect – Or It Should, Anyway!
The difference between magic and illusion – or one of them, at least – is that someone capable of performing magic is presumed to have the powers innately or been bestowed with them. Therefore they can perform the act as though it is second nature, and such a person performing on stage will simply be following what they do naturally. From that point of view, being an illusionist is in many ways more impressive than performing real magic.
The reason for this is that, in order to perform “stage magic”, you need to learn how to make it look real. To use a slang term, you have to sell the trick. Someone watching may know that you aren't really performing magic, but unless they can offer an alternative explanation they would be advised to keep their trap shut. After all, in order to sell the trick, you need to practice to get to the point where the illusion is undetectable.
This is a difficult skill to master. After all, illusions are hard to carry out and usually require concentration. In order to make this look effortless and keep the crowd entertained, it is not beneficial if you stand there looking like you have no idea if it is going to come off. A crowd will get itchy feet if you give the impression of being clueless, so a good performance depends on you being familiar with the trick.
For some of the trickiest illusions, it can take years to have them perfected to a point where they look completely natural. This is one reason why skilled illusionists can charge a great deal of money for live performances.