That the inclusion of this copyrighted content was accidental really has no bearing. What matters is that it was included. Ther are several possible outcomes.
First of all neither you nor anyone else is under any obligation to notify the copyright owner on the character used. If the owner does not become aware of this, there are no consequences.
Te movie maker could redo the scenes where the copyrighted character appears. How much time and expense this would require is highly variable. it is the movie maker's choice whether to do this or not.
The movie maker could contact the copyright owner and ask for permission to use the character. The owner can say yes on whatever terms s/he pleases, including a flat fee or a royalty agreement, or can say no. This involves the movie maker admitting tyhat the copyright owner's work was used.
The copyright owner could send a DMCA takedown notice. (see DMCA counter notice and court action) This is a procedure specific to US law, although some people in other countries use it anyway. Some other countries have a somewhat similar procedure. Thew owner sends a notice to the host. The host may, but need not, honor the notice, by taking down of disabling access to the allegedly infringing content. If the host does not, the host loses legal protection and could be liable for damages if the owner files a copyright suit and wins. The host must notify the user who posted the content. If the user sends a counter notice claiming that the content is not infringing, the host must notify the owner. The owner has 10 days to file suit. If the owner does not do so, the host may restore the content and will still be safe from liability. If the host does not restore the content, the host could be liable to the user if they have a contract requiring the host to post the content. This is normally only relevant if the user is paying the host for hosting services or has some other sort of publishing contract with the host.
If the content is taken down, a revised, non-infringing version could be put back up, or the same version could be put up with permission, if that is obtained.
Both DMCA notices and counter notices are made under penalty of perjury. But in practice, it is rare, almost unheard of, for anyone to be prosecuted for false statements in notices or counter notices, even in pretty blatant cases. a user or host cannot require law enforcement to prosecute such a case.
The copyright owner could file a copyright infringement suit. This costs significant time and money for court fees, legal preparation, and lawyers fees, so most owners will not do it unless they think there is a good chance of getting more money than the costs.
If the owner wins a US copyright suit, the owner could get either actual financial damages, or statutory damages, but not both. Statutory damages could be anywhere between $750 and $30,000 per work infringed, as the court thinks just. If the infringement is proved to be "wilful" the upper limit is $150,000. If the infringer is "innocent" (that is proves that s/he has no reason to believe that the act was one of infringement, and that there was no intention to infringe), the lower limit is $200. The court has wide discretion on what to award for statutory damages, the law only says that the award should be "just".
In addition to damages, the owner could get an injunction (court order) forbidding further distribution of the infringing work(s). In some cases the winner may get legal fees and expenses from the loser in addition.
A major defense, under US law, is often that the use of the work constituted fair use. (see this law.se answer.) Fair use is a specifically US concept in copyright law. It always depends heavily on the specific facts of the use, and there is not nearly enough information in the question here to even start a fair use analysis. Usually a fair use does not have significant economic impact on the copyright owner.
Some EU and other countries have a somewhat similar, but generally narrower, legal concept known as "fair dealing". The Berne Copyright Convention provides for "exceptions so copyright". Both fair use and fair dealing are examples of such exceptions.