See @schoen's good explanation to solve the issue with a computer.
It can be solved with a dictionary as well, by looking up the character from its pronunciation.
With neither computer nor dictionary at hand, it would mostly fall into three scenarios. First, you vaguely remember the character, but can't produce the correct one. Then you can make up a character that look closely to it, like an approximation ("entrepenor") in your example. People will get what you mean from the context.
Some people will substitute with another character of the same sound. Usually it is a character of much simpler form. This is actually an evolving process naturally occurred in history (I don't know what is the correct term in linguistics.) Many very complicated characters had been gradually replaced by simple ones of the same sound in Chinese's >2000 years' history. That is one of the reasons why one character may have multiple meanings.
Some other people will substitute with pinyin. It mostly happens in children's writing. Many children start writing diary at very early age (Chinese is indeed a difficult written language, so we have to start early :) Children like to try new words, especially the "big" words they hear from adults. But it is tiring and frustrating to keep looking up dictionary. Pinyin comes to the rescue. I find it cute, even aesthetic visually, that a child's writing has occasionally pinyin mixed with characters.