Many have contemplated breaking the law in large or small ways. Perhaps a regular retail customer notes that the cash register always has large amounts of money. It could be an accountant realizing that it would be easy to redirect funds to a personal account in an untraceable manner.
Perhaps the theoretical plan makes it far enough where the supervillain discusses their plan with others. It could be a hypothetical discussion with friends or co-workers over a few drinks. Pushing this scenario further, say a waiter at the bar overheard the discussion and reported it to the police. Can they arrest the budding mastermind? No crime was yet committed.
Is there a conspiracy to commit a crime?
Generally speaking, people discussing a hypothetical plan for committing a crime will not get arrested. It changes if there is an attempt to commit the crime, but if there is no robbery or fraudulent accounting (successful or attempted), there is no actual crime.
The situation escalates if the idea person conspires to commit a crime by finding a willing partner to execute the plan could make it a crime and bring it closer to criminal behavior, but still likely will not lead to a conviction unless they broke other laws in the process. This is why two of the four men who conspired to kidnap the Michigan governor were acquitted, but they still face weapons and explosive charges.
There could be charges
Those who are facing felony or misdemeanor charges involving conspiracy or talking about a hypothetical committing a crime need to take these charges seriously, regardless of whether they believe they committed no criminal activity. Clearing themselves of all charges enables them to continue enjoying the freedoms that are their right.